1918: Spanish Flu

Twenty to 40 percent of the world's population became ill, and more than 50 million people died. Many died quickly. Young adults were at high risk; the attack rate and mortality were highest among adults 20 to 50 years old. The severity of that virus has not been seen again.

1957: Asian Flu

The virus was quickly identified in February of that year. Vaccine was available in limited supply by August. By December, the worst seemed to be over. Early in 1958, however, there was another wave of illness among the elderly. In all, about 70,000 people in the United States died.

1968: Hong Kong Flu

The illness was detected in early 1968 but did not become widespread in the United States until December; about 34,000 died in late 1968 and early 1969. Those over 65 were most likely to succumb. The virus returned in 1970 and 1972.

1976: Swine Flu Threat

When first identified, it was labeled the "killer flu." A pandemic was feared, leading to a major vaccination campaign in the United States. But the virus never spread.

1977: Russian Flu Threat

In May 1977, viruses in northern China spread rapidly, causing epidemic disease in those under age 23 worldwide. By January 1978, the virus had spread to the United States.

1997: Avian Flu Threat

Six died from avian flu in Hong Kong. All chickens in Hong Kong (about 1.5 million) were slaughtered. The avian flu did not easily spread from person to person, and after the poultry slaughter, no new human infections were found.

1999: Avian Flu Threat

Another virus caused illnesses in two children in Hong Kong.

What Is Swine Flu?

Swine flu is an influenza virus, like the strains that cause such misery to people during the winter months. Flu viruses also infect many other species of mammals, as well as birds, and this strain causes a respiratory disease in pigs.

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