General information

Image: negative stained transmission electron migrograph of recreated 1918 influenza virions, courtesy CDC photo library.

General Information and Advice

Pandemics arise when a new virus emerges against which the human population has little or no immunity and which is capable of spreading in the worldwide population. This can result in several, simultaneous epidemics worldwide with enormous numbers of deaths and illness. This was the situation during the influenza pandemic of 1918-19, when a completely new influenza virus subtype (influenza A/H1N1) emerged and spread around the globe in around four to six months. Several waves of infection occurred over two years, killing an estimated 40-50 million people. Since then there have been three subsequent influenza pandemics; in 1957 (A/H2N2), 1968 (A/H3N2) and influenza A(H1N1)pdm09 in 2009.

All three of the pandemics in the 20th century were caused by new subtypes of influenza that were probably formed by combination of genes from both avian and human influenza viruses. The recent influenza A(H1N1)pdm09 influenza pandemic was not caused by a new subtype but a triple reassorted swine influenza virus common in the US with genetic material from Eurasian swine flu. The influenza A(H1N1)pdm09 pandemic virus was first identified in humans in Mexico in 2009 as a mild/asymptomatic disease in the majority of cases but severe illness and death in a small proportion of cases, particularly in more vulnerable groups. The threat of a more severe and disrupting pandemic along the lines of the 20th century pandemics remains and the UK has to be prepared for such an event.