Social history of viruses

The social history of viruses describes the influence of viruses and viral infections on human history. Epidemics caused by viruses began when human behaviour changed during the Neolithic period, around 12,000 years ago, when humans developed more densely populated agricultural communities. This allowed viruses to spread rapidly and subsequently to become endemic. Viruses of plants and livestock also increased, and as humans became dependent on agriculture and farming, diseases such as potyviruses of potatoes and rinderpest of cattle had devastating consequences.

The Neolithic age, which began in the Middle East in about 9500 BC, was a time when humans became farmers.^[9] This agricultural revolution embraced the development of monoculture and presented an opportunity for the rapid spread of several species of plant viruses.^[10] The divergence and spread of sobemoviruses – southern bean mosaic virus – date from this time.^[11] The spread of the potyviruses of potatoes, and other fruits and vegetables, began about 6,600 years ago.^[10]

About 10,000 years ago the humans who inhabited the lands around the

Other, more ancient, viruses have been less of a threat. Herpes viruses first infected the ancestors of modern humans over 80 million years ago.^[17] Humans have developed a tolerance to these viruses, and most are infected with at least one species.^[18] Records of these milder virus infections are rare, but it is likely that early hominids suffered from colds, influenza and diarrhoea caused by viruses just as humans do today. More recently evolved viruses cause epidemics and pandemics – and it is these that history records.^[17]

In antiquity

Among the earliest records of a viral infection is an Egyptian

A measles infection confers lifelong immunity. Therefore, the virus requires a high population density to become endemic, and this probably did not occur in the Neolithic age.^[25] Following the emergence of the virus in the Middle East, it reached India by 2500 BC.^[29] Measles was so common in children at the time that it was not recognised as a disease. In Egyptian hieroglyphs it was described as a normal stage of human development.^[30] One of the earliest descriptions of a virus-infected plant can be found in a poem written by the Japanese Empress Kōken (718–770), in which she describes a plant in summer with yellowing leaves. The plant, later identified as Eupatorium lindleyanum, is often infected with tomato yellow leaf curl virus.^[31]

Middle Ages

The rapidly growing population of Europe and the rising concentrations of people in its towns and cities became a fertile ground for many infectious and contagious diseases, of which the Black Death – a bacterial infection – is probably the most notorious.^[32] Except for smallpox and influenza, documented outbreaks of infections now known to be caused by viruses were rare. Rabies, a disease that had been recognised for over 4000 years,^[33] was rife in Europe, and continued to be so until the development of a vaccine by Louis Pasteur in 1886.^[34] The average life expectancy in Europe during the Middle Ages was 35 years; 60% of children died before the age of 16, many of them during their first 6 years of life. Physicians – what few there were – relied as much on astrology as they did on their limited medical knowledge. Some treatments for infections consisted of ointments prepared from cats that had been roasted in hedgehog fat.^[35] Among the plethora of diseases that caused childhood death were measles, influenza and smallpox.^[36] The Crusades and the Muslim conquests aided the spread of smallpox, which was the cause of frequent epidemics in Europe following its introduction to the continent between the fifth and seventh centuries.^[37][38]

Measles was endemic throughout the highly populated countries of Europe, North Africa and the Middle East.^[39] In England the disease, then called "mezils", was first described in the 13th century, and it was probably one of the 49 plagues that occurred between 526 and 1087.^[29] Rinderpest, which is caused by a virus closely related to measles virus, is a disease of cattle known since Roman times.

References to influenza infections date from the late 15th and early 16th centuries,[53] but infections almost certainly occurred long before then.^[54] In 1173, an epidemic occurred that was possibly the first in Europe, and in 1493, an outbreak of what is now thought to be swine influenza, struck Native Americans in Hispaniola. There is some evidence to suggest that source of the infection was pigs on Columbus's ships.^[55] During an influenza epidemic that occurred in England between 1557 and 1559, five per cent of the population – about 150,000 – died from the infection. The mortality rate was nearly five times that of the 1918–19 pandemic.^[47] The first pandemic that was reliably recorded began in July 1580 and swept across Europe, Africa, and Asia.^[56] The mortality rate was high – 8,000 died in Rome.^[57] The next three pandemics occurred in the 18th century, including that during 1781–82, which was probably the most devastating in history.^[58] This began in November 1781 in China and reached Moscow in December.^[57] In February 1782 it hit Saint Petersburg, and by May it had reached Denmark.^[59] Within six weeks, 75 per cent of the British population were infected and the pandemic soon spread to the Americas.^[60]

Aztec drawings of people with smallpox (above) and measles (below)

The Americas and Australia remained free of measles and smallpox until the arrival of European colonists between the 15th and 18th centuries.[1] Along with measles and influenza, smallpox was taken to the Americas by the Spanish.^[1] Smallpox was endemic in Spain, having been introduced by the Moors from Africa.^[61] In 1519, an epidemic of smallpox broke out in the Aztec capital Tenochtitlan in Mexico. This was started by the army of Pánfilo de Narváez, who followed Hernán Cortés from Cuba and had an African slave with smallpox aboard his ship.^[61] When the Spanish finally entered the capital in the summer of 1521, they saw it strewn with the bodies of smallpox victims.^[62] The epidemic, and those that followed during 1545–1548 and 1576–1581, eventually killed more than half of the native population.^[63] Most of the Spanish were immune; with his army of fewer than 900 men it would not have been possible for Cortés to defeat the Aztecs and conquer Mexico without the help of smallpox.^[64] Many Native American populations were devastated later by the inadvertent spread of diseases introduced by Europeans.^[1] In the 150 years that followed Columbus's arrival in 1492, the Native American population of North America was reduced by 80 per cent from diseases, including measles, smallpox and influenza.^[65][66] The damage done by these viruses significantly aided European attempts to displace and conquer the native population.^[67][68]

By the 18th century, smallpox was endemic in Europe. There were five epidemics in London between 1719 and 1746, and large outbreaks occurred in other major European cities. By the end of the century about 400,000 Europeans were dying from the disease each year.

In 1546 Girolamo Fracastoro (1478–1553) wrote a classic description of measles. He thought the disease was caused by "seeds" (seminaria) that were spread from person to person. An epidemic hit London in 1670, recorded by Thomas Sydenham (1624–1689), who thought it was caused by toxic vapours emanating from the earth.^[29] His theory was wrong but he was a skilled observer and kept meticulous records.^[71]

Yellow fever is an often lethal disease caused by a flavivirus. The virus is transmitted to humans by mosquitoes (Aedes aegypti) and first appeared over 3,000 years ago.^[72] In 1647, the first recorded epidemic occurred on Barbados and was called "Barbados distemper" by John Winthrop, who was the governor of the island at the time. He passed quarantine laws to protect the people – the first ever such laws in North America.^[73] Further epidemics of the disease occurred in North America in the 17th, 18th and 19th centuries.^[74] The first known cases of dengue fever occurred in Indonesia and Egypt in 1779. Trade ships brought the disease to the US, where an epidemic occurred in Philadelphia in 1780.^[75]

Newly emerging infectious diseases (EIDs) have been posing an increasingly significant threat to human health. The majority of EIDs are of zoonotic origin,^[76] for which human population increase and the intensification of animal farming as well as of wild animal environments are causative in part.^[77][78]

Many paintings can be found in the museums of Europe depicting

Until the Irish Great Famine of 1845–1852, the commonest cause of disease in potatoes was not the mould that causes blight, it was a virus. The disease, called "curl", is caused by potato leafroll virus, and it was widespread in England in the 1770s, where it destroyed 75 per cent of the potato crop. At that time, the Irish potato crop remained relatively unscathed.^[82]

Discovery of vaccination

Smallpox

Lady

The experiment was repeated on eleven orphan children, all of whom survived the ordeal, and by 1722 even King George I's grandchildren had been inoculated.^[86] The practice was not entirely safe and there was a one in fifty chance of death.^[87] The procedure was expensive; some medical practitioners charged between £5 and £10 and some sold the method to other practitioners for fees between £50 and £100, or for half of the profits. Variolation became a lucrative franchise, but it remained beyond the means of many until the late 1770s.^[88] At the time nothing was known about viruses or the immune system, and no one knew how the procedure afforded protection.^[89]

Edward Jenner (1749–1823), a British rural physician, was variolated as a boy.^[90] He had suffered greatly from the ordeal but survived fully protected from smallpox.^[91] Jenner knew of a local belief that dairy workers who had contracted a relatively mild infection called cowpox were immune to smallpox. He decided to test the theory (although he was probably not the first to do so).^[92] On 14 May 1796 he selected "a healthy boy, about eight years old for the purpose of inoculation for the Cow Pox".^[93] The boy, James Phipps (1788–1853), survived the experimental inoculation with cowpox virus and developed only a mild fever. On 1 July 1796, Jenner took some "smallpox matter" (probably infected pus) and repeatedly inoculated Phipps's arms with it. Phipps survived and was subsequently inoculated with smallpox more than 20 times without succumbing to the disease. Vaccination – the word is derived from the Latin vacca meaning "cow" – had been invented.^[94] Jenner's method was soon shown to be safer than variolation, and by 1801 more than 100,000 people had been vaccinated.^[95]

Despite objections from those medical practitioners who still practised variolation, and who foresaw a decline in their income, free vaccination of the poor was introduced in the UK in 1840. Because of associated deaths, variolation was declared illegal in the same year.^[95] Vaccination was made compulsory in England and Wales by the 1853 Vaccination Act, and parents could be fined £1 if their children were not vaccinated before they were three months of age. The law was not adequately enforced, and the system for providing vaccinations, unchanged since 1840, was ineffective. After an early compliance by the population only a small proportion were vaccinated.^[96] Compulsory vaccination was not well received and, following protests, the Anti-Vaccination League and the Anti-Compulsory Vaccination League were formed in 1866.^[97][98] Following the anti-vaccination campaigns there was a severe outbreak of smallpox in Gloucester in 1895, the city's first in twenty years; 434 people died, including 281 children.^[99] Despite this, the British government conceded to the protesters and the Vaccination Act of 1898 abolished fines and made provision for a "conscientious objector" clause – the first use of the term – for parents who did not believe in vaccination. During the following year, 250,000 objections were granted, and by 1912 less than half of the population of newborns were being vaccinated.^[100] By 1948, smallpox vaccination was no longer compulsory in the UK.^[101]

Rabies

Rabies is an often fatal disease caused by the infection of mammals with rabies virus. In the 21st century it is mainly a disease that affects wild mammals such as foxes and bats, but it is one of the oldest known virus diseases: rabies is a Sanskrit word (rabhas) that dates from 3000 BC,^[34] which means "madness" or "rage",^[30] and the disease has been known for over 4000 years.^[33] Descriptions of rabies can be found in Mesopotamian texts,^[102] and the ancient Greeks called it "lyssa" or "lytta", meaning "madness".^[33] References to rabies can be found in the Laws of Eshnunna, which date from 2300 BC. Aristotle (384–322 BC) wrote one of the earliest undisputed descriptions of the disease and how it was passed to humans. Celsus, in the first century AD, first recorded the symptom called hydrophobia and suggested that the saliva of infected animals and humans contained a slime or poison – to describe this he invented the word "virus".^[33] Rabies does not cause epidemics, but the infection was greatly feared because of its terrible symptoms, which include insanity, hydrophobia and death.^[33]

In France during the time of Louis Pasteur (1822–1895) there were only a few hundred rabies infections in humans each year, but cures were desperately sought. Aware of the possible danger, Pasteur began to look for the "microbe" in mad dogs.^[103] Pasteur showed that when the dried spinal cords from dogs that had died from rabies were crushed and injected into healthy dogs they did not become infected. He repeated the experiment several times on the same dog with tissue that had been dried for fewer and fewer days, until the dog survived even after injections of fresh rabies-infected spinal tissue. Pasteur had immunised the dog against rabies, as he later did with 50 more.^[104]

Although Pasteur had little idea how his method worked, he tested it on a boy, Joseph Meister (1876–1940), who was brought to Pasteur by his mother on 6 July 1885. He was covered in bites, having been set upon by a mad dog. Meister's mother begged Pasteur to help her son. Pasteur was a scientist, not a physician, and he was well aware of the consequences for him if things were to go wrong. He nevertheless decided to help the boy and injected him with increasingly virulent rabid rabbit spinal tissue over the following 10 days.^[105] Later Pasteur wrote, "as the death of this child appeared inevitable, I decided, not without deep and severe unease ... to try out on Joseph Meister the procedure, which had consistently worked on dogs".^[106] Meister recovered and returned home with his mother on 27 July. Pasteur successfully treated a second boy in October that same year; Jean-Baptiste Jupille (1869–1923) was a 15-year-old shepherd boy who had been severely bitten as he tried to protect other children from a rabid dog.^[107] Pasteur's method of treatment remained in use for over 50 years.^[108]

Little was known about the cause of the disease until 1903 when

At the turn of the 20th century, evidence for the existence of viruses was obtained from experiments with filters that had pores too small for bacteria to pass through; the term "filterable virus" was coined to describe them.^[110] Until the 1930s most scientists believed that viruses were small bacteria, but following the invention of the electron microscope in 1931 they were shown to be completely different, to a degree that not all scientists were convinced they were anything other than accumulations of toxic proteins.^[111] The situation changed radically when it was discovered that viruses contain genetic material in the form of DNA or RNA.^[112] Once they were recognised as distinct biological entities they were soon shown to be the cause of numerous infections of plants, animals and even bacteria.^[113]

Of the many diseases of humans that were found to be caused by viruses in the 20th century one, smallpox, has been eradicated. The diseases caused by viruses such as HIV and influenza virus have proved to be more difficult to control.

Smallpox virus was a major cause of death in the 20th century, killing about 300 million people.^[128] It has probably killed more humans than any other virus.^[129] In 1966 an agreement was reached by the World Health Assembly (the decision-making body of the World Health Organization) to start an "intensified smallpox eradication programme" and attempt to eradicate the disease within ten years.^[130] At the time, smallpox was still endemic in 31 countries^[131] including Brazil, the whole of the Indian sub-continent, Indonesia and sub-Saharan Africa.^[130] This ambitious goal was considered achievable for several reasons: the vaccine afforded exceptional protection; there was only one type of the virus; there were no animals that naturally carried it; the incubation period of the infection was known and rarely varied from 12 days; and infections always gave rise to symptoms, so it was clear who had the disease.^[132][133]

Following mass vaccinations, disease detection and containment were central to the eradication campaign. As soon as cases were detected, they were isolated as were their close contacts, who were vaccinated.

Before the September 11 attacks on the United States in 2001, the World Health Organization proposed the destruction of all the known remaining stocks of smallpox virus that were kept in laboratories in the US and Russia.^[139] Fears of bioterrorism using smallpox virus and the possible need for the virus in the development of drugs to treat the infection have put an end to this plan.^[140] Had the destruction gone ahead, smallpox virus might have been the first to be made extinct by human intervention.^[141]

Measles

Measles was a rare – although most often fatal – infection in South Africa in the early nineteenth century but epidemics increased in frequency from the 1850s. During the

Before the introduction of vaccination in the US in the 1960s there were more than 500,000 cases each year resulting in about 400 deaths. In developed countries children were mainly infected between the ages of three and five years old, but in developing countries half the children were infected before the age of two.[143] In the US and the UK, there were regular annual or biannual epidemics of the disease, which depended on the number of children born each year.^[144] The current epidemic strain evolved in the first part of the 20th century – probably between 1908 and 1943.^[145]

In London between 1950 and 1968 there were epidemics every two years, but in Liverpool, which had a higher birth rate, there was an annual cycle of epidemics. During the Great Depression in the US before the Second World War the birth rate was low, and epidemics of measles were sporadic. After the war the birth rate increased, and epidemics occurred regularly every two years. In developing countries with very high birth rates, epidemics occurred every year.^[144] Measles is still a major problem in densely populated, less-developed countries with high birth rates and lacking effective vaccination campaigns.^[146]

By the mid-1970s, following a mass vaccination programme that was known as "make measles a memory", the incidence of measles in the US had fallen by 90 per cent.^[147] Similar vaccination campaigns in other countries have reduced the levels of infection by 99 per cent over the past 50 years.^[148] Susceptible individuals remain a source of infection and include those who have migrated from countries with ineffective vaccination schedules, or who refuse the vaccine or choose not to have their children vaccinated.^[149] Humans are the only natural host of measles virus.^[147] Immunity to the disease following an infection is lifelong; that afforded by vaccination is long term but eventually wanes.^[150]

The use of the vaccine has been

During the summers of the mid-20th century, parents in the US and Europe dreaded the annual appearance of poliomyelitis (or polio), which was commonly known as "infantile paralysis".^[158] The disease was rare at the beginning of the century, and worldwide there were only a few thousand cases per year, but by the 1950s there were 60,000 cases each year in the US alone^[159] and an average of 2,300 in England and Wales.^[160]

During 1916 and 1917 there had been a major epidemic in the US; 27,000 cases and 6,000 deaths were recorded, with 9,000 cases in New York City.^[161] At the time nobody knew how the virus was spreading.^[162] Many of the city's inhabitants, including scientists, thought that impoverished slum-dwelling immigrants were to blame even though the prevalence of the disease was higher in the more prosperous districts such as Staten Island – a pattern that had also been seen in cities like Philadelphia.^[163] Many other industrialised countries were affected at the same time. In particular, before the outbreaks in the US, large epidemics had occurred in Sweden.^[164]

The reason for the rise of polio in industrialised countries in the 20th century has never been fully explained. The disease is caused by a virus that is passed from person to person by the

Following the development of vaccines in the mid-1950s, mass vaccination campaigns took place in many countries.[168] In the US, after a campaign promoted by the March of Dimes, the annual number of polio cases fell dramatically; the last outbreak was in 1979.^[169] In 1988 the World Health Organization along with others launched the Global Polio Eradication Initiative, and by 1994 the Americas were declared to be free of disease, followed by the Pacific region in 2000 and Europe in 2003.^[170] At the end of 2012, only 223 cases were reported by the World Health Organization. Mainly poliovirus type 1 infections, 122 occurred in Nigeria, one in Chad, 58 in Pakistan and 37 in Afghanistan. Vaccination teams often face danger; seven vaccinators were murdered in Pakistan and nine in Nigeria at the beginning of 2013.^[171] In Pakistan, the campaign was further hampered by the murder on 26 February 2013 of a police officer who was providing security.^[172]

AIDS

The human immunodeficiency virus (

HIV crossed the species barrier between chimpanzees and humans in Africa in the early decades of the 20th century.[180] During the years that followed there were enormous social changes and turmoil in Africa. Population shifts were unprecedented as vast numbers of people moved from rural farms to the expanding cities, and the virus was spread from remote regions to densely populated urban conurbations.^[181] The incubation period for AIDS is around 10 years, so a global epidemic starting in the early 1980s is credible.^[182] At this time there was much scapegoating and stigmatisation.^[183] The "out of Africa" theory for the origin of the HIV pandemic was not well received by Africans, who felt that the "blame" was misplaced. This led the World Health Assembly to pass a 1987 resolution, which stated that HIV is "a naturally occurring [virus] of undetermined geographic origin".^[184]

The HIV pandemic has challenged communities and brought about social changes throughout the world.^[185] Opinions on sexuality are more openly discussed. Advice on sexual practices and drug use – which were once taboo – is sponsored by many governments and their healthcare providers.^[186] Debates on the ethics of provision and cost of anti-retroviral drugs, particularly in poorer countries, have highlighted inequalities in healthcare and stimulated far-reaching legislative changes.^[187] In developing countries the impact of HIV/AIDS has been profound; key organisations such as healthcare, defense and civil services have been severely disrupted.^[188] Life expectancy has fallen. In Zimbabwe, for example, life expectancy was 79 years in 1991 but by 2001 it had fallen to 39 years.^[189]

Influenza

When influenza virus undergoes a

In 1957 another new strain of the virus emerged and caused a pandemic of Asian flu; although the virus was not as virulent as the 1918 strain, over one million died worldwide. The next pandemic occurred when Hong Kong flu emerged in 1968, a new strain of the virus that replaced the 1957 strain.^[197] Affecting mainly the elderly, the 1968 pandemic was the least severe, but 33,800 were killed in the US.^[198] New strains of influenza virus often originate in East Asia; in rural China the concentration of ducks, pigs, and humans in close proximity is the highest in the world.^[199]

The most recent pandemic occurred in 2009, but none of the last three has caused anything near the devastation seen in 1918. Exactly why the strain of influenza that emerged in 1918 was so devastating is a question that still remains unanswered.^[191]

Yellow fever, dengue and other arboviruses

Arboviruses are viruses that are transmitted to humans and other vertebrates by blood-sucking insects. These viruses are diverse; the term "arbovirus" – which was derived from "arthropod-borne virus" – is no longer used in formal taxonomy because many species of virus are known to be spread in this way.^[200] There are more than 500 species of arboviruses, but in the 1930s only three were known to cause disease in humans: yellow fever virus, dengue virus and Pappataci fever virus.^[201] More than 100 of such viruses are now known to cause human diseases including encephalitis.^[202]

Yellow fever is the most notorious disease caused by a flavivirus.

First appearing in East Africa in 1891, rinderpest, a disease of cattle, spread rapidly across Africa.[228] By 1892, 95 per cent of the cattle in East Africa had died. This resulted in a famine that devastated the farmers and nomadic people, some of whom were entirely dependent on their cattle. Two thirds of the population of Maasai people died. The situation was made worse by epidemics of smallpox that followed in the wake of the famine.^[229] In the early years of the 20th century rinderpest was common in Asia and parts of Europe.^[230] The prevalence of the disease was steadily reduced during the century by control measures that included vaccination.^[231] By 1908 Europe was free from the disease. Outbreaks did occur following the Second World War, but these were quickly controlled. The prevalence of the disease increased in Asia, and in 1957 Thailand had to appeal for aid because so many buffaloes had died that the paddy fields could not be prepared for rice growing.^[232] Russia west of the Ural Mountains remained free from the disease – Lenin approved several laws on the control of the disease – but cattle in eastern Russia were constantly infected with rinderpest that originated in Mongolia and China where the prevalence remained high.^[233] India controlled the spread of the disease, which had retained a foothold in the southern states of Tamil Nadu and Kerala, throughout the 20th century,^[234] and had eradicated the disease by 1995.^[235] Africa suffered two major panzootics in the 1920s and 1980s.^[236] There was a severe outbreak in Somalia in 1928 and the disease was widespread in the country until 1953. In the 1980s, outbreaks in Tanzania and Kenya were controlled by the use of 26 million doses of vaccine, and a recurrence of the disease in 1997 was suppressed by an intensive vaccination campaign.^[237] By the end of the century rinderpest had been eradicated from most countries. A few pockets of infection remained in Ethiopia and Sudan,^[238] and in 1994 the Global Rinderpest Eradication Programme was launched by the Food and Agriculture Organization (FAO) with the aim of global eradication by 2010.^[239] In May 2011, the FAO and the World Organisation for Animal Health announced that "rinderpest as a freely circulating viral disease has been eliminated from the world."^[240]

Foot-and-mouth disease is a highly contagious infection caused by an aphthovirus, and is classified in the same family as poliovirus. The virus has infected animals, mainly ungulates, in Africa since ancient times and was probably brought to the Americas in the 19th century by imported livestock.^[241] Foot-and-mouth disease is rarely fatal, but the economic losses incurred by outbreaks in sheep and cattle herds can be high.^[242] The last occurrence of the disease in the US was in 1929, but as recently as 2001, several large outbreaks occurred throughout the UK and thousands of animals were killed and burnt.^[243]

The natural hosts of influenza viruses are pigs and birds, although it has probably infected humans since antiquity.^[244] The virus can cause mild to severe epizootics in wild and domesticated animals.^[245] Many species of wild birds migrate and this has spread influenza across the continents throughout the ages. The virus has evolved into numerous strains and continues to do so, posing an ever-present threat.^[246]

In the early years of the 21st century epizootics in livestock caused by viruses continue to have serious consequences. Bluetongue disease, a disease caused by an orbivirus broke out in sheep in France in 2007.^[247] Until then the disease had been mainly confined to the Americas, Africa, southern Asia and northern Australia, but it is now an emerging disease around the Mediterranean.^[248]

Plant viruses

In the 1920s the

Even without mutation, it is always possible that some hitherto obscure parasitic organism may escape its accustomed ecological niche and expose the dense populations that have become so conspicuous a feature of the earth to some fresh and perchance devastating mortality. McNeill (1998) p. 293

Emerging viruses are those that have only relatively recently infected the host species.

Severe acute respiratory syndrome (SARS) is caused by a new type of coronavirus.^[264] Other coronaviruses were known to cause mild infections in humans,^[265] so the virulence and rapid spread of this novel virus strain caused alarm among health professionals as well as public fear.^[260] The fears of a major pandemic were not realised, and by July 2003, after causing around 8,000 cases and 800 deaths, the outbreak had ended.^[266] The exact origin of the SARS virus is not known, but evidence suggests that it came from bats.^[267]

A related coronavirus emerged in

West Nile virus, a flavivirus, was first identified in 1937 when it was found in the blood of a feverish woman. The virus, which is carried by mosquitoes and birds, caused outbreaks of infection in North Africa and the Middle East in the 1950s and by the 1960s horses in Europe were also affected. The largest outbreak in humans occurred in 1974 in Cape Province, South Africa and 10,000 people became ill.^[278] An increasing frequency of epidemics and epizootics (in horses) began in 1996, around the Mediterranean basin, and by 1999 the virus had reached New York City. Since then the virus has spread throughout the US.^[278] In the US, mosquitoes carry the highest amounts of virus in late summer, and the number of cases of the disease increases in mid July to early September. When the weather becomes colder, the mosquitoes die and the risk of disease decreases.^[279] In Europe, many outbreaks have occurred; in 2000 a surveillance programme began in the UK to monitor the incidence of the virus in humans, dead birds, mosquitoes and horses.^[280] The mosquito (Culex modestus) that can carry the virus breeds on the marshes of north Kent. This mosquito species was not previously thought to be present in the UK, but it is widespread in southern Europe where it carries West Nile virus.^[281]

Nipah virus

In 1997 an outbreak of respiratory disease occurred in Malaysian farmers and their pigs. More than 265 cases of encephalitis, of which 105 were fatal, were recorded.^[282] A new paramyxovirus was discovered in an individual's brain; it was named Nipah virus, after the village where he had lived. The infection was caused by a virus from fruit bats, after their colony had been disrupted by deforestation. The bats had moved to trees nearer the pig farm and the pigs caught the virus from their droppings.^[283]

Viral haemorrhagic fevers

Several highly lethal viral pathogens are members of the Filoviridae. Filoviruses are filament-like viruses that cause viral haemorrhagic fever, and include the Ebola and Marburg viruses. The Marburg virus attracted widespread press attention in April 2005 after an outbreak in Angola. Beginning in October 2004 and continuing into 2005, there were 252 cases including 227 deaths.^[284]

The

The natural source of Ebola virus is probably bats.[289]^[290] Marburg viruses are transmitted to humans by monkeys,^[291] and Lassa fever by rats (Mastomys natalensis).^[292] Zoonotic infections can be severe because humans often have no natural resistance to the infection and it is only when viruses become well-adapted to new host that their virulence decreases. Some zoonotic infections are often "dead ends", in that after the initial outbreak the rate of subsequent infections subsides because the viruses are not efficient at spreading from person to person.^[293]

The beginning of the 21st century saw an increase in the global awareness of devastating epidemics in developing countries, which, in previous decades had passed relatively unnoticed by the international health community.^[294]

Beneficial viruses

Sir Peter Medawar (1915–1987) described a virus as "a piece of bad news wrapped in a protein coat".^[295] With the exception of the bacteriophages, viruses had a well-deserved reputation for being nothing but the cause of diseases and death. The discovery of the abundance of viruses and their overwhelming presence in many ecosystems has led modern virologists to reconsider their role in the biosphere.^[296]

It is estimated that there are about 10³¹ viruses on Earth. Most of them are bacteriophages, and most are in the oceans.^[297] Microorganisms constitute more than 90 per cent of the biomass in the sea,^[298] and it has been estimated that viruses kill approximately 20 per cent of this biomass each day and that there are fifteen times as many viruses in the oceans as there are bacteria and archaea.^[298] Viruses are the main agents responsible for the rapid destruction of harmful algal blooms, which often kill other marine life,^[298] and help maintain the ecological balance of different species of marine blue-green algae,^[299] and thus adequate oxygen production for life on Earth.^[300]

The emergence of strains of bacteria that are resistant to a broad range of antibiotics has become a problem in the treatment of bacterial infections.

The Human Genome Project has revealed the presence of numerous viral DNA sequences scattered throughout the human genome.^[308] These sequences make up around eight per cent of human DNA,^[309] and appear to be the remains of ancient retrovirus infections of human ancestors.^[310] These pieces of DNA have firmly established themselves in human DNA.^[308] Most of this DNA is no longer functional, but some of these viruses have brought with them novel genes that are important in human development,^{[311][312][313]} and could aid the immune system's B cells destroy tumours.^[314] Viruses have transferred important genes to plants. About ten per cent of all photosynthesis uses the products of genes that have been transferred to plants from blue-green algae by viruses.^[315]

References