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FRIDAY, July 18 (HealthDayNews) -- A tiny but growing number of people are becoming infected with two strains of the AIDS virus, spurring concerns about whether this extremely unusual condition may become more common.

So-called "superinfection" can disrupt drug treatment in AIDS patients and may complicate research into a vaccine to prevent transmission in the first place.

"There's a potential impact if it gets out of hand," said Dr. Margaret Fischl, director of the AIDS Clinical Research Unit at the University of Miami School of Medicine.

She added that superinfection can occur when the virus from one HIV-positive person infects another. "It reinforces the need to assume responsibility for yourself and therefore others so you don't expose yourself to another strain of virus," she said.

Doctors reported at least three cases of superinfection last year, the first to appear in the history of the AIDS epidemic. The cases raised fears that HIV-positive patients could get secondary infections by viruses that are immune to the drugs they're already taking.

Drug resistance is an increasing problem among AIDS patients. The virus in their bodies can mutate and develop immunity to specific classes of medications, forcing doctors to try other drugs.

The latest reports of superinfection came this week at an international AIDS conference in Paris. According to news reports, a doctor from Geneva reported finding superinfection in two Swiss intravenous drug users, while another doctor from Albany, N.Y., said two strains had combined to create a blended strain in a female patient.

That kind of "hybrid" presents a special challenge to researchers who are trying to develop an AIDS vaccine. They may be able create a vaccine that protects people against a subtype "A" and subtype "B," but fighting off a combination of A-B would be even harder, Fischl said. "That would be the worst scenario that we're looking at," she added.

The problem is that hybrid viruses are less vulnerable to attack from a vaccine, according to Frank Myers, an epidemiologist and AIDS expert at Scripps Mercy Hospital in San Diego.

"This doesn't mean the vaccine would be useless," he said. "We have vaccines that are only effective against some strains of a bacteria, like the pneumococcal vaccine is an example of this. But it would mean that you are not protected against all strains of the agent."

Both Fischl and Myers emphasized that the findings support the importance of protected sex, even between HIV-positive people.

More information

To learn more about HIV/AIDS and treatments, visit the Centers for Disease Control and Prevention or the National Institute of Allergy and Infectious Diseases.