Flu pandemics occur whenever 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 different H types and 9 different N types, but numbers merely differentiate the strains and do not indicate greater severity.

In the 20th century, there were three flu pandemics. Global health officials are carefully watching the ongoing swine flu outbreaks in Mexico and the U.S. to see if they might spark the next one.

A glance at the last three pandemics:

• 1918. The Spanish flu pandemic that started in 1918 was possibly the deadliest outbreak of all time. It was first identified in the U.S., 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 1918 flu was an H1N1 strain – different from the one currently affecting Mexico and the U.S. – and struck mostly healthy young adults. Experts estimate it killed about 40 to 50 million people worldwide.

• 1957. The 1957 pandemic was known as the Asian flu. It was sparked by an H2N2 strain and was first identified in China. There were two waves of illness during this pandemic; the first wave mostly hit children while the second mostly affected the elderly. It caused about 2 million deaths globally.

• 1968. The most recent pandemic, known as the Hong Kong flu, was the mildest of the three pandemics this century. It was first spotted in Hong Kong in 1968 and it spread globally over the next two years. The people most susceptible to the virus were the elderly. About 1 million people are estimated to have been killed by this pandemic, an H3N2 flu strain.

AP-ES-04-25-09 1326EDT