WASHINGTON (AP) _ Harmless forms of cholera bacteria are turned into killers when they are infected by a virus that switches on a poison-making gene, according to new research.

The true villain in cholera is a bacteriophage _ a virus that infects bacteria. When this virus infects the cholera bacteria it installs a gene that converts the harmless organism into a toxin factory. That change floods the guts of victims with a poison that can cause a lethal diarrhea, said Dr. Matthew K. Waldor of Tufts University.

``When people get infected with a vibrio (cholera bacteria) that already has that gene, then they can get sick,'' Waldor said in an interview. ``If they ingest a vibrio that didn't have the virus, then they most likely wouldn't get sick.''

A report on the research by Waldor and Dr. John J. Mekalanos of the Harvard Medical School is being published Friday in the journal Science.

Experts said the study demonstrates a fundamental new way that virus genes can use bacteria to cause illness.

``It's one of the most important findings in cholera research in the last 10 years,'' Stephen Richardson of Wake Forest University said in a statement to Science.

In the study, Waldor and Mekalanos showed that a virus can link up with cholera bacteria that have on their cell surface a structure called pili. The virus uses the pili to move into the bacteria cell, where it inserts a new gene into the bacteria chromosome.

This gene becomes a part of the bacteria genetic structure and creates a strain of cholera that can cause disease, said Waldor.

Cholera causes disease by producing a toxin, or poison, that triggers a very serious diarrhea.

``Patients can lose up to 20 liters (5.2 gallons) a day,'' said Waldor.

He said only the strains of cholera with both the pili structure and the toxin gene activated by the virus are dangerous.

The new understanding of cholera may lead eventually to a vaccine that would block the action of the virus gene, and, thus, prevent the disease, said Waldor.

Cholera usually is spread through infected drinking water. There have been seven worldwide epidemics since the 1870s. More than 200,000 people were infected in the early 1990s in Southeast Asia. The disease is rare in the United States, where public water supplies are monitored and usually treated.