Common cat fleas are the culprits in spreading a species of bacteria in cats that causes a rare disease in some people when they are scratched by infected cats, according to UC researchers' findings. Their study explains the prevalence of the bacterium in the cat population, currently estimated to be nearly 60 million cats in U.S. households. Infected cats display no symptoms and appear to be unharmed by the organism. In humans, on the other hand, the bacterium is responsible for 22,000 cases of 'cat scratch' disease annually in the country. Infected people with healthy immune systems show skin irritation, swollen nymph nodes and a slight fever. For those with weakened immune systems, the disease can cause serious and sometimes life-threatening problems. Transmission from fleas to humans has not been documented and requires further study, according to the researchers from UC San Francisco and UC Davis. Their results appear in this month's issue of the Journal of Clinical Microbiology. Both UC Davis epidemiologist Bruno Chomel of the School of Veterinary Medicine and Jane Koehler, an assistant professor of medicine at UC San Francisco, discourage mass screening and treating of infected cats. Although there appears to be a huge number of cats infected, they say, the number of people who get infected is relatively small and the infections are typically benign for those with healthy immune systems.