Two conditions must be satisfied for an outbreak of influenza to be classed as a pandemic. Firstly, the outbreak of infection, arising in a specific geographical area, spreads throughout the world; a high percentage of individuals are infected resulting in increased mortality rates. Secondly, a pandemic is caused by a new influenza virus A subtype, the HA of which is not related to that of influenza viruses circulating immediately before the outbreak, and could not have arisen from those viruses by mutation (Webster and Laver 1972). Each influenza A virus possesses one of 15 distinct HA molecules, designated H1, H2, H3, and so on, which do not cross-react in serological tests: as stated above, immunity to influenza is principally related to antibody to the HA (Hobson et al. 1972), and the appearance of a new virus subtype with a different HA means that immunity acquired from past influenza infection confers no protection against the new virus subtype, and the spread of infection by the latter is unchecked. This is a totally different phenomenon to antigenic drift, and is termed antigenic shift.