International health experts are increasingly concerned that the avian influenza that has infected birds in East Asia, Southeast Asia, Russia and Kazakhstan could cause a global pandemic.  But the bird flu is just one of a number of potentially lethal diseases challenging the international medical community.  Leta Hong Fincher looks at how the United States and other countries are trying to respond.

In the recent Hollywood movie, "War of the Worlds," aliens attack the United States, causing widespread panic. Some fear this is the kind of panic that could break out if the United States were hit with a sudden epidemic.

Chuck Ludlum has recently worked on biodefense policy issues in the U.S. Congress and he's worried about the future. "We need to have an imagination that that's exactly what we could see if avian flu came to the United States and it had a lethality rate of 55 percent. We would see public panic on a scale that is hard to imagine."

Mr. Ludlum argues that the United States is unprepared for either a bioterrorist attack or a sudden outbreak of a new disease like avian influenza.

Since late 2003, nearly 100 million birds around the world have either died of the virus or been killed to prevent the epidemic from spreading. Yet the bird flu continues to spread from countries like China, Thailand and Vietnam to Russia and Kazakhstan.

Fearing the worst, governments in Europe are already taking steps to prevent a bird flu outbreak in their countries. The Dutch government has ordered poultry farmers to move millions of normally free-ranging birds indoors.

And the European Commission has met in Brussels to discuss what coordinated action its 25 member nations can take against the bird flu. Medical experts believe it is only a matter of time before the bird virus changes into a form that is transmitted easily by humans.

Dr. William Raub is a U.S. public health emergency official at the Department of Health and Human Services."If we get the combination of something that spreads easily from person to person, as the regular flu does, but also is highly lethal, as this avian strain is, then we have all the makings for another global pandemic," says Dr. Raub.

The bird flu is not the only potentially lethal disease to challenge the medical community. The AIDS epidemic continues to expand globally, with a record 40 million people now infected with the HIV virus. In hardest-hit southern Africa, more than a quarter of the population is infected.

Health experts say ultimately, an AIDS vaccine offers the best hope for stopping the HIV virus.  But progress in finding a vaccine has been slow.

Helene Gayle, head of HIV initiatives at the Bill and Melinda Gates Foundation in Seattle, Washington, feels, "The challenge of creating this HIV vaccine will be a marathon. It's not a sprint."

But while AIDS at least has an effective antiretroviral treatment, other diseases have no cure at all.  In Africa, Ebola and Marburg viruses kill more than 90 percent of their victims, often within a week.

And there are as many as 300,000 new cases of drug-resistant tuberculosis a year, with patients in former Soviet bloc countries most likely to be infected with a drug-resistant "superstrain."

With today's high-speed transportation, diseases are spreading around the world faster than ever. Dr. Raub points to the Severe Acute Respiratory Syndrome, or SARS, epidemic of 2003 as a classic example.

"We found ourselves with a disease we had not seen before, an organism we had not seen before,? Dr Raub said. ?We therefore had no diagnostics, no drugs, and no vaccines. But we had it spreading rapidly around the world because of the ease of international travel."

Some health experts argue that pharmaceutical companies need more incentives to develop drugs and vaccines against a natural epidemic or bioterrorist attack.

Anthony Fauci directs the U.S. government's infectious disease research.  He doesn?t think this approach will work. "It is not a great financial incentive for drug companies to get involved in making vaccines and drugs for microbes that we may not ever have to use these vaccines or drugs for."

Last year, the Bush administration approved a "Bioshield" program of more than $5 billion in incentives to private companies to develop drugs and vaccines that work against a disease outbreak or bioterrorist attack.  And Congress is now debating a "Bioshield II" bill that would further boost the country's biopreparedness efforts.

Still, the pace of progress remains troubling to some medical experts, who believe it is virtually certain that at some point, a major epidemic for which no treatment exists will strike... again.