Dr. Donald Burke has been working on an AIDS vaccine since 1986.

"At the time I started," said the dean of the University of Pittsburgh's Graduate School of Public Health, "the usual question was, 'How long is it going to be before we have an AIDS vaccine?'

"So being conservative, I said, 'It'll probably take 10 years.' So 1996 came around, and by then, I was still saying 10 years, and now 2006 has come and gone and I don't say anything anymore because it has proven much more difficult than we imagined."

A recent article in Scientific American suggested that an effective AIDS vaccine has been hard to produce because the virus mutates at an incredible rate and directly attacks immune system cells that would normally help fight it.

Dr. Burke, who spent much of his career as an infectious disease specialist in the military, said both those things are true, but that still doesn't explain why a vaccine has been so elusive.

After all, flu viruses mutate constantly, he said, and yet scientists have figured out ways to immunize people against the flu.

"If I knew the answer, we would have solved the problem," Dr. Burke said. "And so if you find anybody who says they know the answer, they don't."

The one thing he does know is that in the world of infectious diseases, it is the slow, chronic infections that are the most challenging for which to create vaccines.

That is true whether the disease comes from a virus (AIDS), bacteria (tuberculosis) or a parasite (malaria).

"The chronic diseases are chronic because they have found ways to evade the immune system or have mechanisms to actively alter or suppress or divert the immune system," Dr. Burke said.

HIV, the virus that causes AIDS, changes once it gets inside a person's body, so the virus the immune system is initially trained to attack is not the same one it sees days or weeks later.

And tuberculosis, like a Trojan horse, gets inside the immune system cells that are designed to kill the bacteria and converts them into factories that provide the bacteria with nutrients, a group of French scientists reported this month.

Vaccines work best against diseases that tend to hit people's bodies hard and then leave, Dr. Burke said, such as flu or measles.

The struggle to come up with an AIDS vaccine contrasts sharply with the progress that has been made in diagnosing and treating the disease, he said.

In industrialized nations, doctors now can tell how much of the virus an infected person has, what major subtype it belongs to, and then can prescribe drugs that keep the patient alive, if not completely healthy.

But a vaccine would be the least expensive way of stopping the epidemic and would be far and away the best treatment in poorer nations, he said.

After smallpox was eradicated with a vaccine more than 30 years ago, one of the leaders of that effort told Dr. Burke that "we recoup the total cost of that eradication in less than two months today," based on savings in present-day treatment costs.

"As the dean of a school of public health, I genuinely believe an ounce of prevention is worth a pound of cure."

One reason Americans haven't been up in arms over the lack of an AIDS vaccine is that the main burden of the disease today is falling on other people, particularly in Africa and Asia.

Because of drug treatments, people falsely believe the epidemic is under control, but "the notion that we've got it solved couldn't be a worse misrepresentation of what's going on," Dr. Burke said. "There are millions of new infections a year around the world, and even in the United States, we have 40,000 to 50,000 new cases a year."

The same complacency exists in America and Europe toward other tropical scourges, like dengue fever, which Dr. Burke also has specialized in.

Dengue (pronounced DEN-gay) is a viral infection spread by mosquitoes that causes a flu-like fever, and in severe cases, can cause vomiting, bleeding and death, mostly in children. It infects an estimated 50 million people a year in tropical zones.

There are four strains of the dengue virus. The first two were identified in soldiers during World War II by Dr. Albert Sabin, who later developed the oral polio vaccine. The other two were identified in the same building Dr. Burke works in today at the University of Pittsburgh, by a team led by Dr. William Hammon.

Dr. Burke, who helped start work that is currently under way in Thailand to develop a vaccine against dengue, said there are several reasons why Americans should pay more attention to such diseases.

American military personnel are frequently sent into parts of the world with these diseases. Expanding global travel and migration also mean that more and more people can come into contact with these bugs.

Finally, it is hard to know when a disease will mutate in animals or humans so that it spreads rapidly inside our borders, he said. "The West Nile virus sat in Africa for 60 years and then all of a sudden it swept through the United States unannounced," he noted.

AIDS, which has ballooned from a few thousand cases to tens of millions, provides a sobering reminder that the best way to stop a global pandemic is to snuff it out before the flames can spread, and that is increasingly where Dr. Burke is focusing his energies.

Most epidemics start with germs that make the leap from animals to humans when people come in contact with the animals' body fluids, he said.

AIDS probably began when monkey viruses infected African bush meat hunters, he said, and the recent avian flu outbreaks crossed over from infected chickens and ducks to humans.

Once animal germs have infected humans, they go through an adaptation phase before they become virulent enough to begin spreading rapidly through the population, he said, and that is the critical time to monitor infections and prepare to stifle them.

Using a supercomputer, Dr. Burke and his colleagues have simulated what would happen if an avian flu epidemic began in densely populated Thailand. By modeling how all 85 million people in the region interact with each other, the computer program estimated that 95 percent of the spread could be halted by using 300,000 courses of antiviral drugs.

As a result of such calculations, Roche has donated more than 5 million treatment courses of its drug Tamiflu to the World Health Organization for such emergencies.

Pitt's Graduate School of Public Health currently has a $10 million grant from the Bill and Melinda Gates Foundation to improve its computer modeling of infectious disease epidemics.

"If we can monitor the viral chatter -- it's like terrorism chatter, when you monitor the airwaves and you know something is happening -- and we can see a virus is jumping species, that's where we should be positioning ourselves to quench epidemics before they begin."

In Dr. Burke's office at Pitt, a magnified satellite photo of the Earth sits beneath the glass surface of a circular table. Occasionally, he likes to wrap his arms around the photo, cradling the globe.

It's a symbolic way of acknowledging to himself how interconnected all of us are.