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 pandemic. As of 2018, nearly 38 million people worldwide are estimated to be infected with HIV, with 1.8 million new infections occurring each year. AIDS has caused around 32 million deaths. The major routes of transmission are unsafe heterosexual or male/male sex, contaminated needles and blood transfusions, and from mother to child, either at birth or via breastfeeding. Combination antiretroviral therapy does not eliminate the virus, but delays progression to AIDS and can lead to a near-normal life expectancy. Drugs to prevent infection are now available, but there is no effective HIV vaccine.

The first immortal human cell line, HeLa cells were derived from a cervical cancer biopsy and carry human papillomavirus 18 DNA. The cells have been growing since 1951.

Credit: Gerry Shaw (8 March 2012)

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

RNA interference is a type of gene silencing that forms an important part of the immune response against viruses and other foreign genetic material in plants and many other eukaryotes. A cell enzyme called Dicer (pictured) cleaves double-stranded RNA molecules found in the cell cytoplasm – such as the genome of an RNA virus or its replication intermediates – into short fragments termed small interfering RNAs (siRNAs). These are separated into single strands and integrated into a large multi-protein RNA-induced silencing complex, where they recognise their complementary messenger RNA (mRNA) molecules and target them for destruction. This prevents the mRNAs acting as a template for translation into proteins, and so inhibits, or silences, the expression of viral genes.

RNA interference allows the entire plant to respond to a virus after a localised encounter, as the siRNAs can transfer between cells via plasmodesmata. The protective effect can be transferred between plants by grafting. Many plant viruses have evolved elaborate mechanisms to suppress this response. RNA interference evolved early in eukaryotes, and the system is widespread. It is important in innate immunity towards viruses in some insects, but relatively little is known about its role in mammals. Research is ongoing into the application of RNA interference to antiviral treatments.

The 1993 hantavirus outbreak in the Four Corners region of southwest USA was of a novel hantavirus, subsequently named Sin Nombre virus. It caused the previously unrecognised hantavirus pulmonary syndrome – the first time that a hantavirus had been associated with respiratory symptoms. Mild flu-like symptoms were followed by the sudden onset of pulmonary oedema, which was fatal in half of those affected. A total of 24 cases were reported in April–May 1993, with many of those affected being from the Navajo Nation territory. Hantavirus infection of humans generally occurs by inhaling aerosolised urine and faeces of rodents, in this case the deer mouse (Peromyscus; pictured).

Previously documented hantavirus disease had been confined to Asia and Europe, and these were the first human cases to be recognised in the USA. Subsequent investigation revealed undiagnosed cases dating back to 1959, and Navajo people recalled similar outbreaks in 1918, 1933 and 1934.

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