In the 20th century, the world experienced three influenza A pandemics: “Spanish flu” claiming 20-40 million lives in 1918-9 and the “Asian flu” of 1957 and “Hong Kong flu” of 1968, each of which claimed 1-4 million lives.1 It might be about to face another.

Birds are the natural hosts of influenza A, but most avian viruses are not transmitted to humans. However, the current influenza A/H5N1 virus is more virulent in birds than in the past and is associated with human infections.2 Since its appearance in Hong Kong in 1997, the H5N1 epizootic, affecting both wild birds and domestic poultry, has spread to most countries in South East Asia and recently to Russia and Kazakhstan, directly threatening Europe.3 4 An epidemic of another less virulent virus, A/H7N7, in the Netherlands in 2003 emphasised the potential for emergence of infection in Europe.5

There are three prerequisites for a pandemic: a novel virus subtype for which humans are immunologically naive must be transmitted to humans; it must replicate and cause disease; and it must be efficiently transmitted among humans. …