Viruses made the leap from birds to humans with catastrophic results three times in the 20th century. The three avian influenza pandemic are:
- Spanish Flu 1918-19
The [A (H1N1)] virus caused the highest number of known flu deaths (40-50 million worldwide). Many died within a few days of the infection. Others died of complications soon after. Nearly half the fatalities were young, healthy adults.
- Asian Flu 1957-58
This [A (H2N2)] virus killed about one million people. First identified in China in late February 1957, it had spread to the United States by June that year.
- Hong Kong Flu 1968-69
Type A (H3N2) virus circulates even today. The mortality rate in this epidemic was lower than in earlier ones because many humans had developed Asian flu antibodies, which helped fight this mutation. First seen in Hong Kong in early 1968, the virus had spread to the US by later that year.