With nearly three-quarters of a billion people traveling internationally every year and millions ... Travel spreads risk of inf

With nearly three-quarters of a billion people traveling internationally every year and millions more relocating as immigrants and refugees, infectious disease is no longer a local issue, but a global concern.

As the international health community marks World AIDS Day, there is increasing evidence that at least some of these re-emerging infections are being driven by the AIDS pandemic.

When HIV rates rise, so do rates of opportunistic infections, whether bacterial, viral, parasitic or fungal. TB is the most common re-emerging infection to be spread by HIV and accounts for 15 percent of all AIDS deaths worldwide, according to the World Health Organization. To date, some 25 million people have died of AIDS around the globe, and in 2005 the number of people living with HIV in the United States has reached its highest level yet at 1.2 million.

Many other opportunistic infections, like AIDS itself, are zoonotic in origin, which means they can pass from animals to humans. Scientists now believe that HIV jumped the species barrier some 50 to 70 years ago when hunters in Cameroon slaughtered chimpanzees and became infected with the primates' contaminated blood.

Today, doctors in Western countries, especially the United States and southwestern Europe, are seeing more AIDS patients with unusual tropical infections.

"HIV patients always present new challenges," said Margaret Neill, an infectious disease specialist at Brown University Medical School and the director of the travel clinic at Memorial Hospital in Providence, R.I.

And when they do, says Neill, they often expose themselves to unusual pathogens. In a healthy person these diseases are ordinarily not problematic, but because HIV attacks the body's natural defenses, AIDS patients are more vulnerable to opportunistic infections.

One of those unusual infections is visceral leishmaniasis (leash-ma-NY-a-sis), a parasitic pathogen. Spread principally from dogs to humans, and then person-to-person, by the female sand fly, the symptoms of visceral leishmaniasis include high fever, drenching sweats, cough, fatigue, weight loss and severe abdominal pain. There are about a half-million cases reported each year and if left untreated, the mortality rate is nearly 100 percent.

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