Drug-resistant versions of the human immunodeficiency virus are also a growing problem. People develop resistance while taking AIDS drugs, but then this resistant virus can be passed from one person to another.

Persaud's team studied 21 HIV-infected infants in 10 U.S. states. They found five of them were infected with drug-resistant HIV from their mothers.

The virus moved quickly to so-called resting or inactive CD4 T-cells -- the cells normally infected by HIV, Persaud's team reported in The Journal of Infectious Diseases.

The virus was resistant to a class of AIDS drugs called non-nucleoside reverse transcriptase inhibitors or NNRTIs. But protease inhibitors, another class of AIDS drugs, worked, the researchers found.

"The initial transmitted drug-resistant virus will likely never be cleared from that infant with currently available treatments," Persaud said in a statement.

The AIDS virus is especially difficult to fight because it infects the immune system cells that usually battle a viral infection. Single drugs do very little to control it, so it is important to use cocktails of drugs that interfere with the virus at various points in its life cycle.

There are currently about 20 different available AIDS drugs in various classes, but patients with resistant virus automatically cannot use several of them.

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