Epidemics of influenza occur in most countries in some years, and in some countries in most years: for many they are common annual events, unpredictable in time and severity. However, history indicates certain features which make influenza epidemics more likely. Firstly, epidemics tend to occur in winter months when cold, crowding of people and higher humidities are a feature: indeed, in areas where continuous high humidity is a characteristic, infections may occur throughout the year. Secondly, in more recent times epidemics are often first seen in Eastern or Southern Hemisphere countries, and later spread to Europe and North America in the winter months of these areas: seeding of infection may occur prior to epidemics, to be followed by epidemics when conditions are optimal. Thirdly, epidemics are more likely to occur when a variant virus appears which shows antigenic changes from previous strains, and cross-reacting antibody, acquired by previous infection, is low in both percentage positive and titre. The influenza virus has radiating from the surface multiple copies of two glycoproteins termed haemagglutinin (HA) and neuraminidase (NA): it is antibody to these proteins which equates with immunity, and it is accumulating mutations in these glycoproteins, particularly the haemagglutinin, which constitutes the virus variation termed antigenic drift. The monitoring of antigenic variation in the influenza virus is a key factor in anticipating epidemics, and in vaccine design where for any year the prevalent virus strain(s) must be incorporated into the current vaccines. The responsibility for monitoring virus variation rests with the World Health Organization who have commissioned over a hundred research laboratories in various parts of the world to monitor antigenic changes in the infecting viruses and the incidence and spread of infection.