Researchers say they've developed the first vaccine for visceral leishmaniasis—a disease that affects 500,000 people each year and has been called the "parasitic version of HIV."

The vaccine took researchers more than two decades to develop and entered Phase I trials in recent weeks, according to Steve Reed, founder of the Infectious Disease Research Institute, the vaccine's developer.

Visceral leishmaniasis infects victims' internal organs and bone marrow and destroys blood cells. Without treatment, the disease has a 90 percent mortality rate and can kill within two years. The disease has been called the parasitic version of HIV--though the two illnesses are unrelated--because of the similar way it attacks a victim's immune system.

It is caused by a bite from an infected sandfly, tiny creatures whose bites are painless. About 50,000 people die from the disease each year, making it the second most common parasitic killer in the world, after malaria.

Most current treatments for the disease, including chemotherapy, are costly and toxic to the patient. The vaccine, Reed says, will likely cost just pennies per patient and may help eradicate the disease entirely.

"The problem is some of the treatments are quite toxic, and no disease can be eliminated based on treatment alone," Reed says. "You need to prevent it entirely, and if we have fewer humans that are infected, we'll have fewer carriers ... I think it can be eradicated."

If the Phase I trials are successful, Reed says testing will be expanded in 2013 before eventual distribution in countries such as India, Nepal, and Bangladesh, where the disease is most prevalent.

Unlike HIV, where a potential vaccine has eluded scientists for years because the virus mutates very rapidly, the organism that causes leishmaniasis remains constant, so researchers have always had hope for developing a vaccine. But the protozoan responsible is much more complex than a virus, making vaccine development a tough nut to crack.