Pandemics arise when a new virus emerges which is capable of spreading in the worldwide population. Unlike ordinary seasonal influenza that occurs every winter in the UK, pandemic flu can occur at any time of the year.
The most devastating influenza pandemic in recorded history occurred in 1918-19, when a completely new influenza virus subtype (influenza A/H1N1, "Spanish flu") 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. Two further influenza pandemics occurred in the 20th century: in 1957 (influenza H2N2, "Asian flu") and in 1968 (influenza H3N2, "Hong Kong flu").
Most recently the influenza A (H1N1) 'swine flu' pandemic virus emerged in Mexico in 2009 causing mild/asymptomatic disease in the majority of cases but severe illness and death in a small proportion of cases, particularly in more vulnerable groups.