Cats share their bugs with humans, too.

Cats are everywhere. More than one-third of U.S. households own a cat, and health care professionals sometimes advise the sick and elderly to seek out feline companionship.

A new study, however, may make cat owners wish they had never adopted their beloved puss. Physicians from the University of California, San Francisco, report that people, particularly those with weak immune systems, can contract a serious bacterial infection from their meowing mouser. But the news isn't all bad: A course of antibiotics usually gets rid of it in both humans and cats, Jane E. Koehler and her colleagues report.

These researchers examined four patients diagnosed with a bacterial infection called bacillary angiomatosis (BA). All had had contact with cats. When Koehler's group ran blood tests on the cats, they found that the animals were infected with Rochalitnaea henselae, the bacterium that causes BA.

While the humans suffered from skin lesions, fever, and other complications, their feline friends showed no symptoms. Three of the humans, who had advanced HIV infection, were particularly vulnerable to the bacterium, Koehler and her colleagues write in the Feb. 16 JOURNAL OF THE AMERICAN MEDICAL ASSOCIATION.

The physicians also found R. henselae in 25 of 61 additional pet or stray cats they examined. They even detected DNA from R. henselae in one cat's fleas. All of the patients had numerous flea bites, which suggests that these bugs may help transmit the bacterium, the authors contend.

Researchers have recently implicated Rochalimaea bacteria as the cause of trench fever and cat scratch disease, two other human illnesses.