Influenza is caused by a virus called the influenza virus. It is spread by tiny droplets carried in the air when people who are unwell with the influenza infection cough, sneeze or talk. They may be contagious up to 1 day before they develop any symptoms. The virus can also be spread by touching contaminated surfaces such as doorknobs and telephones.
The influenza virus has several strains and the ability to mutate rapidly. This causes seasonal changes in the prevalent strain of influenza infection. In most temperate countries, occurrence of influenza peaks once a year during winter. In Singapore, influenza occurs all year round with 2 peak flu seasons, one between December and February (during the Northern Hemisphere winter) and the other between May and July (during the Southern Hemisphere winter).
Occasionally, there are influenza pandemics of certain strains which cause more severe diseases all over the world. Some examples include the avian flu, as also known as influenza A (H5N1), in 1997 and swine flu, also known as influenza A (H1N1), in 2009.