How Much of a Threat Is Bird Influenza? Influenza, with its
fever, aches, fatigue and threat of complications, seems a
uniquely human illness. But the flu, caused by a virus, can
infect animals and birds, as well. The good news is that many
strains of the flu virus don't cross over to humans. The
sobering news is that the few strains that do make the crossover
tend to be virulent, with the potential to cause widespread
illness. That's what happened in 1918, when the "Spanish flu"
swept around the world, killing at least 40 million people. That
was the worst death toll of any cause in history. And it was
caused by a virulent strain of the flu virus that infected both
birds and humans. Nearly 30 years later, in 1946, Thomas Francis
and Jonas Salk (who later developed the polio vaccine) produced
the first vaccine against the flu. Despite the vaccine's
availability, influenza pandemics occurred again in 1957-58
(Asian flu), 1968-69 (Hong Kong flu) and 1977 (Russian flu).
Many people worldwide died in these pandem