Flu pandemics occur when a new strain of the virus arises to which most people have no natural immunity. Experts believe they have been occurring at irregular intervals since at least the 16th century.
Scientists classify flu viruses according to their surface proteins: H for hemagluttinin and N for neuraminidase. There are 16 H types and 9 N types, but numbers merely differentiate the strains and do not indicate greater severity.
The 20th century had three flu pandemics:
1918: The Spanish flu pandemic was possibly the deadliest outbreak of all time. It was first identified in the United States but became known as the Spanish flu because it received more media attention in Spain than in other countries, which were censoring the press during World War I. The flu was an H1N1 strain, different from the one currently affecting Mexico and the United States, and it struck mostly healthy young adults. Experts say it killed up to 50 million people worldwide.
1957: The Asian flu pandemic was sparked by an H2N2 strain and was first identified in China. There were two waves of illness: The first wave hit children while the second affected the elderly. It caused about 2 million deaths worldwide.
1968: The Hong Kong flu was first spotted in Hong Kong in 1968, and it spread globally over the next two years. The elderly were most susceptible to the virus. About 1 million people were killed by the pandemic, an H3N2 flu strain.