Portal:Viruses/Selected article

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Portal:Viruses/Selected article/1

skin rash that is similar to measles. Rarely, the disease develops into the life-threatening dengue haemorrhagic fever, resulting in bleeding, low levels of blood platelets and blood plasma leakage, or into dengue shock syndrome, where dangerously low blood pressure

occurs.
Dengue is transmitted by several species of
immunity to that type but only short-term immunity to the others, and subsequent infection with a different type increases the risk of severe complications. Treatment of acute dengue is supportive, using oral or intravenous fluids for mild or moderate disease; blood transfusion may be required in more severe cases. A vaccine against dengue fever
is available. Reducing mosquito habitats and limiting exposure to bites are the other main methods of control.

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Child with a deformed right leg due to poliomyelitis

Bulbar involvement is rare, but in severe cases the virus can prevent breathing by affecting the phrenic nerve, so that patients require mechanical ventilation with an iron lung
or similar device.
Depictions in ancient art show that the disease has existed for thousands of years. The virus was an
eradication campaign
started in 1988. The annual incidence of wild-type disease fell from many hundreds of thousands to 22 in 2017, but has since resurged to a few hundreds.

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Child with smallpox rash, showing the characteristic raised blisters with a central depression

blindness resulting from corneal ulceration and scarring, and in young children, limb deformities due to arthritis and osteomyelitis
.
Smallpox probably emerged in human populations in about 10,000 BC; the mummified body of Egyptian pharaoh
Ramses V shows evidence of smallpox rash. The disease was responsible for an estimated 300–500 million deaths during the 20th century. Smallpox vaccine, the earliest vaccine, was developed in the 18th century. After intensive vaccination campaigns, the last natural infection occurred in 1977. Smallpox was certified the first infectious disease to be eradicated
globally in 1979. Debate is ongoing over whether all stocks of the virus should be destroyed.

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Symptoms of influenza

fatigue. Influenza is occasionally associated with nausea and vomiting, particularly in children. Pneumonia
is a rare complication which can be life-threatening.
Influenza spreads around the world in seasonal epidemics, resulting in about 3–5 million cases of severe illness annually, and about 250,000–500,000 deaths, mainly in the young, the old and those with other health problems. Annual influenza vaccinations are recommended for those at high risk. Sporadic influenza pandemics have been recorded since at least the 16th century. The Spanish flu pandemic of 1918–20 is estimated to have killed 50–100 million people.

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Light microscope image of an H&E-stained liver biopsy, showing "ground glass hepatocytes" associated with chronic hepatitis B infection

body fluids. Mother-to-child transmission is a major route in endemic countries. HBV is 50–100 times more infectious than HIV. The virus replicates in liver cells, and enters the blood where viral proteins and antiviral antibodies
are found.
Acute infection is often asymptomatic but can cause
chronic carriers. The virus is endemic in East Asia and sub-Saharan Africa. Infection can be prevented by vaccination
.

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Cold sore on the lower lip (arrow)

herpesviral encephalitis and Mollaret's meningitis
.
After initial infection, virus particles are transported along
Neonatal herpes is possible after transmission from the mother. Barrier protection methods reduce genital herpes risk. No vaccine or cure exists, but antiviral
treatment can alleviate symptoms and reduce viral shedding.

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British World War 2 poster, stressing the economic cost of the common cold

The
Ebers papyrus
, the oldest surviving medical text, written before the 16th century BCE.
Symptoms include
contaminated objects. The viruses that cause colds can survive for prolonged periods in the environment (over 18 hours for rhinoviruses). Hand washing can help to prevent spread. No effective antiviral treatment or vaccine
currently exists.

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Shingles rash on the chest

Shingles, or herpes zoster, is a painful skin rash with blisters that, characteristically, occurs in a stripe limited to just one side of the body. The rash usually heals within 2–5 weeks, but around one in five people experience residual nerve pain for months or years.
Shingles is caused by
dermatome
. Exactly how VZV remains latent in the body, and subsequently reactivates, is unclear.
Around a third of the population will develop shingles. Repeated episodes are rare. In the United States, about half the cases occur in people aged 50 years or older. Vaccination at least halves the risk, and prompt treatment with aciclovir or related antiviral drugs can reduce the severity and duration of the rash.

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Diagram showing the three layers of the meninges: the dura mater (blue), arachnoid mater (green) and pia mater (fawn)

nonspecific symptoms
, such as irritability or drowsiness.
The most common cause is infection with
fungi, protozoa and parasites can also cause meningitis, and there are several non-infectious causes. Although some forms of meningitis can be life-threatening, viral meningitis is generally more benign than that caused by bacterial infection. It usually resolves spontaneously and is rarely fatal. HSV-2 can cause a chronic, recurrent form called Mollaret's meningitis
.
Polymerase chain reaction of cerebrospinal fluid and identification of antibodies can be used to differentiate between viral causes. Viral meningitis typically only requires supportive therapy; meningitis caused by HSV or varicella zoster virus sometimes responds to treatment with antiviral drugs such as aciclovir. Mumps-associated meningitis can be prevented by vaccination.

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blood transfusions in the developing world. In about 80% of those infected, the virus establishes a chronic infection in the liver, and around 10–30% of those infected will develop cirrhosis over 30 years. Some people with cirrhosis go on to develop liver failure, liver cancer
or other serious complications.
An estimated 143 million people worldwide (2%) have chronic HCV infections as of 2015. The prevalence is highest in Central and East Asia, North Africa and the Middle East. The virus causes around a quarter of cases of cirrhosis and
liver transplantation. The recommended therapy is an appropriate combination of protease inhibitors. Although 95% of people treated in this way are cured, the treatments are expensive and older therapies are less effective. No vaccine
against hepatitis C is available.

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Hand washing with soap is a protective measure against gastroenteritis

adenovirus and astrovirus. Other major infectious causes include Campylobacter, Escherichia coli, Vibrio cholerae and some other bacteria, as well as protozoa
. Viruses, particularly rotavirus, cause about 70% of gastroenteritis episodes in children, while norovirus is the leading cause of gastroenteritis among adults in America, causing over 90% of outbreaks.
Transmission can be from consumption of improperly prepared foods or contaminated water, or by close contact with infectious individuals. Good sanitation practices and a convenient supply of uncontaminated water are important for reducing infection. Personal measures such as hand washing with soap can decrease incidence by as much as 30%. An estimated 2 billion cases of gastroenteritis occurred globally in 2015, mainly among children and people in developing countries, resulting in 1.3 million deaths. Gastroenteritis is usually an acute and self-limiting disease that does not require medication; the main treatment is rehydration using oral rehydration therapy. A rotavirus vaccine is available.

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immunity
.
The first definitive outbreak of yellow fever was in
endemic in tropical and subtropical areas of South America and Africa; its incidence has been increasing since the 1980s. An estimated 200,000 cases and 30,000 deaths occur each year, with almost 90% of cases being in Africa. Antiviral therapy is not effective. A vaccine is available, and vaccination, mosquito control
and bite prevention are the main preventive measures.

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Chickenpox rash in an adult male

respiratory route
, as well as direct contact with lesions.
A classic disease of childhood, the highest prevalence occurs at 4–10 years. Chickenpox is rarely fatal in people with a normal immune system, with around 6,400 deaths worldwide in 2015, about 1 in 60,000 infections. Adults often have more severe symptoms than children, and are at higher risk of complications such as pneumonia, bronchitis, hepatitis and encephalitis. Pregnant women and people with a suppressed immune system have the highest complication risk. Chickenpox during the first 28 weeks of gestation can lead to foetal malformations. Infection in adults is usually treated with antiviral drugs, such as aciclovir or valaciclovir, which reduces symptom severity and the risk of complications. A vaccine is available.

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Graph of HIV levels (red) and decline in T cells (blue) during progression of HIV/AIDS

AIDS is a progressive immunodeficiency disease first recognised in 1981, caused by infection with human immunodeficiency virus (HIV). Initial infection is sometimes associated with a brief influenza-like illness, followed by a period of 3–20 years without symptoms during which the immune system deteriorates. The individual becomes highly susceptible to common infections such as tuberculosis, as well as opportunistic infections, such as Pneumocystis pneumonia, and tumours, such as Kaposi's sarcoma and lymphoma. Severe weight loss is also a feature of AIDS. Without treatment, the average survival time after infection is around 9–11 years, depending on the HIV subtype.
HIV/AIDS is a global
HIV vaccine
.

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immune suppression, and long-term use of oral contraceptives
.
Cervical cancer is the fourth most common cancer in women worldwide. It can be detected by
HPV vaccines
protect against high-risk virus strains, and can prevent up to 90% of cervical cancers. Where screening and vaccination are not available, cervical cancer has substantial mortality; worldwide, an estimated 569,000 cases and 311,000 deaths occurred in 2018, with around 85% of cases being in developing countries.

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natural host is thought to be megabats. The virus spreads by contact with body fluids, such as blood. After an incubation period of 2–21 days, nonspecific symptoms including fever, sore throat, muscle pain and headache develop, followed by vomiting, diarrhoea, rash, liver and kidney impairment, and sometimes internal or external bleeding. On average, around half of those infected die, often from hypovolaemic shock. The virus sometimes persists in semen or breast milk
for several weeks or months after recovery.
The disease was first identified in 1976 by
bush meat. Ebola vaccines are now available. Monoclonal antibody treatments can improve outcomes, and a combination of three monoclonals
has been licensed.

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psychiatric problems, behavioural changes and painful sensations. Abnormal prion proteins build up as amyloid deposits in the brain, which acquires a characteristic spongiform appearance, with many round vacuoles in the cerebellum and cerebrum. The average life expectancy
after symptoms start is 13 months.
About 170 cases have been recorded in the UK, and 50 cases in the rest of the world. The estimated prevalence in the UK is about 1 in 2000, higher than the reported cases. Transmission is believed to be mainly from consuming beef contaminated with the
PRNP-encoding gene. Human PRNP protein can have either methionine or valine
at position 129; nearly all of those affected had two copies of the methionine-containing form, found in 40% of Caucasians.

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healthcare
.
Measles was first described by
autism
. Rates of disease and deaths increased from 2017 to 2019, attributed to a decrease in immunisation.

Portal:Viruses/Selected article/19

aerosols, semen, consumption of infected food scraps or feed supplements, and via inanimate objects including fodder, farming equipment, vehicles, standing water, and the clothes and skin of humans. Some infected ruminants can transmit infection as asymptomatic carriers
.
Friedrich Loeffler showed the disease to be viral in 1897. FMD was widely distributed in 1945. By 2014, North America, Australia, New Zealand, much of Europe, and some South American countries were free of the disease. Major outbreaks include one in the UK in 2001 that cost an estimated £8 billion. The virus is highly variable, with seven serotypes. A vaccine is available, but protection is temporary and strain specific. Other control methods include monitoring programmes, trade restrictions, quarantine, and the slaughter of infected and healthy at-risk animals.

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aerosols contacting mucous membranes. The typical human incubation period is 1–3 months. The neurotropic virus travels along neural pathways into the CNS and brain, where it causes meningoencephalitis. Nonspecific symptoms such as fever and headache are followed by neurological symptoms, including partial paralysis, confusion, agitation, paranoia, hallucinations and sometimes hydrophobia, which progress to delirium, coma
and death. Around 17,400 people died from rabies in 2015, mainly in Asia and Africa.
Rabies is mentioned in the
immunoglobulin
, is completely effective if begun immediately after exposure, but survival is extremely rare once symptoms have begun.

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mother-to-child transmission can cause microcephaly (pictured) and other brain malformations in some babies. In one study, major abnormalities were seen in 42% of live births. The virus is present in semen and male-to-female sexual transmission
has been documented; it is also found in breast milk and blood.
The first documented human outbreak occurred in 2007 in the
Public Health Emergency of International Concern in February–November 2016. No specific treatment nor vaccine
has been approved. Prevention involves mosquito control and condom use; women in areas where Zika was circulating were recommended to consider delaying pregnancy, and pregnant women were advised to avoid travel to affected areas.

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lice
. It can also be transmitted by direct contact, and is shed in the ocular and nasal discharge and from eroded skin.
Myxoma virus was intentionally introduced in Australia, France and Chile in the 1950s to control wild European rabbit populations. This resulted in short-term 10–100-fold reductions in the rabbit population, followed by its recovery with the emergence of myxomatosis-resistant animals and attenuated virus variants. The introduction of myxomatosis is regarded as a classical example of host–pathogen coevolution following cross-species transmission of a pathogen to a naive host.

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