Influenza

Influenza is a sudden infection of the airways caused by influenza viruses, enveloped RNA viruses of the family Orthomyxoviridae whose genome is split into eight separate segments. Of the four types, A and B cause the seasonal epidemics of winter, C usually causes mild illness and D mainly infects cattle. Influenza A viruses are named after two surface proteins: hemagglutinin (H), which attaches the virus to cells, and neuraminidase (N), which frees new virus particles from them. The subtypes now circulating in people are A(H1N1) and A(H3N2).

Flu spreads in droplets from coughs and sneezes. After a day or two, illness starts suddenly with fever, cough, sore throat, muscle aches and exhaustion. Most people recover within about a week, but pneumonia and death can follow, mainly in older adults, young children, pregnant people and those with chronic illness. Small mutations keep changing the surface proteins, a process called antigenic drift, which is why the vaccine is updated every year. Now and then two strains infecting one host swap whole gene segments, an antigenic shift that can create a new subtype and spark a pandemic, as in 1918 and 2009.

Rapid and molecular tests on a swab from the nose or throat confirm the diagnosis. Antiviral drugs such as oseltamivir, which blocks neuraminidase, can shorten the illness when started early.