Portal:Viruses

Yellow fever is an acute haemorrhagic fever caused by the yellow fever virus, an RNA virus in the Flaviviridae family. It infects humans, other primates, and Aedes aegypti and other mosquito species, which act as the vector. After transmission by the bite of a female mosquito, the virus replicates in lymph nodes, infecting dendritic cells, and can then spread to liver hepatocytes. Symptoms generally last 3–4 days, and include fever, nausea and muscle pain. In around 15% of people, a toxic phase follows with recurring fever, liver damage and jaundice, sometimes accompanied by bleeding and kidney failure; death occurs in 20–50% of those who develop jaundice. Infection otherwise leads to lifelong immunity.

The first definitive outbreak of yellow fever was in Barbados in 1647, and major epidemics have occurred in the Americas and southern Europe since that date. Yellow fever is 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.

1802 cartoon of Edward Jenner administering cowpox vaccine against smallpox, satirising contemporary fears about vaccination.

Credit: James Gillray (12 June 1802)

Map showing the prevalence of SARS-CoV-2 cases; black: highest prevalence; dark red to pink: decreasing prevalence; grey: no recorded cases or no data

26 February: In the ongoing pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), more than 110 million confirmed cases, including 2.5 million deaths, have been documented globally since the outbreak began in December 2019. WHO

18 February: Seven asymptomatic cases of avian influenza A subtype H5N8, the first documented H5N8 cases in humans, are reported in Astrakhan Oblast, Russia, after more than 100,0000 hens died on a poultry farm in December. WHO

14 February: Seven cases of Ebola virus disease are reported in Gouécké, south-east Guinea. WHO

7 February: A case of Ebola virus disease is detected in North Kivu Province of the Democratic Republic of the Congo. WHO

4 February: An outbreak of Rift Valley fever is ongoing in Kenya, with 32 human cases, including 11 deaths, since the outbreak started in November. WHO

21 November: The US Food and Drug Administration (FDA) gives emergency-use authorisation to casirivimab/imdevimab, a combination monoclonal antibody (mAb) therapy for non-hospitalised people twelve years and over with mild-to-moderate COVID-19, after granting emergency-use authorisation to the single mAb bamlanivimab earlier in the month. FDA 1, 2

18 November: The outbreak of Ebola virus disease in Équateur Province, Democratic Republic of the Congo, which started in June, has been declared over; a total of 130 cases were recorded, with 55 deaths. UN

Bats host a diverse array of viruses, including all seven types described by the Baltimore classification system. The most common viruses known to infect bats are coronaviruses. Bats harbour many viruses that are zoonotic, or capable of infecting humans, including rabies virus, SARS-CoV, SARS-CoV-2, MERS-CoV, Nipah virus, Hendra virus and Marburg virus (hosted by the Egyptian fruit bat; pictured), and some bat-borne viruses are considered important emerging viruses. Bats may also play a role in the ecology of the Ebola virus. Most zoonotic bat viruses are transmitted by direct contact with infected bat fluids such as urine, guano and saliva, or through contact with an infected intermediate host; transmission of rabies from bats to humans usually occurs via biting. Butchering or consuming bat meat could potentially lead to viral transmission.

Bats rarely become ill from viral infections, and rabies is the only viral disease known to kill them. They might be more tolerant of infection than other mammals. Their immune systems differ from those of other mammals in their lack of several inflammasomes, which activate the body's inflammatory response, as well as a dampened stimulator of interferon genes response, which helps to control the host response to pathogens.

The West African Ebola epidemic was the most widespread outbreak of the disease to date. Beginning in Meliandou in southern Guinea in December 2013, it spread to adjacent Liberia and Sierra Leone, affecting the cities of Conakry and Monrovia, with minor outbreaks in Mali and Nigeria. Cases reached a peak in October 2014 and the epidemic was under control by late 2015, although occasional cases continued to occur into April 2016. Ring vaccination with the then-experimental vaccine rVSV-ZEBOV was trialled in Guinea.

More than 28,000 suspected cases were reported with more than 11,000 deaths; the case fatality rate was around 40% overall and around 58% in hospitalised patients. Early in the epidemic nearly 10% of the dead were healthcare workers. The outbreak left about 17,000 survivors, many of whom reported long-lasting post-recovery symptoms. Extreme poverty, dysfunctional healthcare systems, distrust of government after years of armed conflict, local burial customs of washing the body, the unprecedented spread of Ebola to densely populated cities, and the delay in response of several months all contributed to the failure to control the epidemic.

Viruses & Subviral agents: bat virome • elephant endotheliotropic herpesvirus • HIV • introduction to viruses • Playa de Oro virus • poliovirus • prion • rotavirus • virus

Diseases: colony collapse disorder • common cold • croup • dengue fever • gastroenteritis • Guillain–Barré syndrome • hepatitis B • hepatitis C • hepatitis E • herpes simplex • HIV/AIDS • influenza • meningitis • myxomatosis • polio • pneumonia • shingles • smallpox

Epidemiology & Interventions: 2007 Bernard Matthews H5N1 outbreak • Coalition for Epidemic Preparedness Innovations • Disease X • 2009 flu pandemic • HIV/AIDS in Malawi • polio vaccine • Spanish flu • West African Ebola virus epidemic

Virus–Host interactions: antibody • host • immune system • parasitism • RNA interference

Methodology: metagenomics

Social & Media: And the Band Played On • Contagion • "Flu Season" • Frank's Cock • Race Against Time: Searching for Hope in AIDS-Ravaged Africa • social history of viruses • "Steve Burdick" • "The Time Is Now" • "What Lies Below"

People: Brownie Mary • Macfarlane Burnet • Bobbi Campbell • Aniru Conteh • people with hepatitis C • HIV-positive people • Bette Korber • Henrietta Lacks • Linda Laubenstein • Barbara McClintock • poliomyelitis survivors • Joseph Sonnabend • Eli Todd • Ryan White