I'm Bob Doughty with the VOA Special English Health Report.

There has not been a worldwide outbreak of influenza sincenineteen sixty-eight. Experts say there should have been another bynow. They hope to be prepared to limit the effects when the next onefinally happens.

The so-called Spanish flu in nineteen eighteen became the mostdeadly influenza pandemic ever recorded. A pandemic is when adisease spreads around the world. It killed an estimated twentymillion to fifty million people. Almost half were young adults.

There were two other flu pandemics in the twentieth century. TheAsian flu struck in nineteen fifty-seven, and the Hong Kong flu innineteen sixty-eight.

Scientists at the United States Centers for Disease Control andPrevention say the cause of the Spanish flu pandemic is not clear.But the two others are known to have resulted from a human virusthat became mixed with an avian influenza virus. And that couldhappen again.

Scientists first identified avian influenza in Italy more thanone hundred years ago. Bird flu is caused by type A influenzaviruses. Type A are the most common, and usually cause the mostserious flu outbreaks in people.

Currently the most serious kind of bird flu is known asa-h-five-n-one. It has spread among chickens and ducks in Asia. Thevirus has infected at least forty-four people in Thailand andVietnam this year. More than thirty of them have died.

Researchers worry that the virus could spread quickly worldwideif it gains the ability to pass easily between people. Manyresearchers say governments must do more to support planning for thenext flu pandemic.

This month, the World Health Organization held a meeting todiscuss efforts to develop a vaccine to prevent infection with thevirus. About fifty experts met in Geneva.

Klaus Stohr heads the global influenza program at the W.H.O. Hesays this is the first chance to produce a vaccine that would limitthe damage caused by a flu pandemic. This is the result ofimprovements in the way scientists study flu outbreaks in people andanimals.

Scientists are developing two vaccines based on the current birdflu virus in Asia. To have both of these "candidate vaccines" testedwithin a year would cost an estimated thirteen million dollars each.Medical experts say a vaccine is unlikely to prevent another flupandemic, but it could save millions of lives.

This VOA Special English Health Report was written by CynthiaKirk. I'm Bob Doughty.