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

There has not been a worldwide outbreak of influenza since 1968. Experts say there should have been another by now. They hope to be prepared to limit the effects when the next one finally happens.

The so-called Spanish flu in 1918 became the most deadly influenza pandemic ever recorded. A pandemic is when a disease spreads around the world. It killed an estimated 20 million to 50 million people. Almost half were young adults.

There were two other flu pandemics in the twentieth century. The Asian flu struck in 1957, and the Hong Kong flu in 1968.

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

Scientists first identified avian influenza in Italy more than 100 years ago. Bird flu is caused by type A influenza viruses. Type A are the most common, and usually cause the most serious flu outbreaks in people.

Currently the most serious kind of bird flu is known as a-h-5-n-1. It has spread among chickens and ducks in Asia. The virus has infected at least 44 people in Thailand and Vietnam this year. More than 30 of them have died.

Researchers worry that the virus could spread quickly worldwide if it gains the ability to pass easily between people. Many researchers say governments must do more to support planning for the next flu pandemic.

This month, the World Health Organization held a meeting to discuss efforts to develop a vaccine to prevent infection with the virus. About 50 experts met in Geneva.

Klaus Stohr heads the global influenza program at the W.H.O. He says this is the first chance to produce a vaccine that would limit the damage caused by a flu pandemic. This is the result of improvements in the way scientists study flu outbreaks in people and animals.

Scientists are developing two vaccines based on the current bird flu virus in Asia. To have both of these "candidate vaccines" tested within a year would cost an estimated 13 million dollars each. Medical experts say a vaccine is unlikely to prevent another flu pandemic, but it could save millions of lives.

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