Scientists at Thomas Jefferson University in Philadelphia labeled as a medical paradox their discovery that AIDS patients whose conditions had been improving thanks to treatment with drug cocktails had been coming under attack from opportunistic infections that ordinarily should not have been much of a problem.

In a study published in the journal Annals of Internal Medicine, the researchers said the sometimes-fatal "immune reconstitution syndrome" stemmed from an inflammatory reaction by the newly strengthened immune system to bacteria or viruses already present in the patient.

The researchers said the causes of the syndrome were unknown.

The researchers said they were startled by the fact that the infections were affecting patients who had been benefiting from so-called highly active antiretroviral therapy (HAART) involving the use of combinations of powerful anti-HIV (human immunodeficiency virus) medicines.

The doctors described learning of patients with a typical infection suffered by those with HIV -- mycobacterium avium infection.

"The infection was not showing up in someone with end-stage AIDS who wasn't taking antiretrovirals and HAART, but someone getting better and on HAART," Thomas Jefferson University's Dr. Joseph DeSimone said in a statement.

DeSimone said some doctors give antibiotics to treat the opportunistic infections, while others prescribe anti-inflammatory drugs. Some doctors reduce the HAART treatment.

"No one is exactly sure what to do against this syndrome yet," DeSimone said.

The study concluded that it would be difficult to form controlled studies to find suppressing agents given the "atypical and sporadic presentation of these reactions" considering the number of patients using drug cocktails.

More than a year ago, researchers began to see patients with HIV, the virus that causes AIDS, developing infections at times that caught them off guard. The Jefferson doctors said they decided to search the medical literature and speak with colleagues to learn whether others had seen similar developments.

They said doctors at other hospitals mentioned infections such as CMV retinitis, an AIDS-related blindness.

"The AIDS-related blindness some patients experience was getting better, and when patients were started on their HAART therapy they overall were getting better," said Dr. Timothy Babinchak, clinical director of infectious diseases at Thomas Jefferson University Hospital.

"But then they seemed to be getting an aggravation of their disease. It was another process, not necessarily the CMV being reactivated. It was an inflammatory reaction to the CMV infection already there."

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