Measles is a disease that only affects humans caused by the measles virus, an RNA virus in the Paramyxoviridae family. It is highly contagious, with transmission occurring via the respiratory route or by contact with secretions. Symptoms generally develop 10–12 days after exposure and last 7–10 days; they include high fever, cough, rhinitis and conjunctivitis, white Koplik's spots inside the mouth and a generalised red maculopapular rash. Complications including diarrhoea, otitis media and pneumonia are relatively common; more rarely seizures, encephalitis, croup, corneal ulceration and blindness can occur. The risk of death is usually around 0.2%, but may be as high as 10–28% in areas with high levels of malnutrition and poor healthcare.

Measles was first described by Rhazes (860–932). The disease is estimated to have killed around 200 million people between 1855 and 2005. It affects about 20 million people a year, primarily in the developing areas of Africa and Asia, and is one of the leading vaccine-preventable disease causes of death. No antiviral drug is licensed. An effective measles vaccine is available, but uptake has been reduced by anti-vaccination campaigns, particularly the fraudulent claim that the MMR vaccine might be associated with autism. Rates of disease and deaths increased from 2017 to 2019, attributed to a decrease in immunisation.

The common cold is the most frequent infectious disease. Despite the advice to "consult your physician" no antiviral treatment has been approved, and colds are only rarely associated with serious complications.

Credit: Federal Art Project (1937)

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

T4 bacteriophage, typical of myovirus bacteriophages

Bacteriophages (or phages) are a large and diverse group of viruses that infect bacteria and archaea. Their genome, which they inject into the host's cytoplasm, can be DNA or RNA, single or double stranded, linear or circular, and contains between four and several hundred genes. Their capsid can be relatively simple or elaborate in structure, and in a few groups is surrounded by an envelope. Caudovirales, double-stranded DNA phages with tails, is the best-studied group, and includes T4 (pictured), λ phage and Mu phage.

Among the most common entities in the biosphere, bacteriophages are ubiquitous in locations populated by bacteria. One of the densest natural sources is sea water, where up to 900 million virions/mL have been found in microbial mats at the surface, and up to 70% of marine bacteria can be infected.

Used as an alternative to antibiotics for over 90 years, phages might offer a potential therapy against multi-drug-resistant bacteria.

In the severe acute respiratory syndrome (SARS) outbreak, the first cases of the newly emerged SARS coronavirus were reported in November 2002 from the Chinese Guangdong province. The virus soon spread across Asia, with China, Hong Kong, Taiwan and Singapore being the worst affected countries; a secondary outbreak occurred in Canada. The rapid initial spread of the outbreak has been in part attributed to China's slow response to the early cases. Over 8,000 people were infected, with a case fatality rate of 11%. Those over 65 years had a much higher mortality rate, greater than 55%. The outbreak was contained by July 2003, and no cases have been reported since 2004.

At the time of the outbreak, the immediate source of SARS coronavirus was thought to have been the masked palm civet (Paguma larvata; pictured), which was sold as food in Guangdong markets. The virus was also found in raccoon dogs, ferret badgers and domestic cats. More recent research has suggested that the natural reservoir could be horseshoe bats.

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