Some of that poop is laden with an infectious parasite known as Toxoplasma gondii, a protozoan that has recently caused toxoplasmosis epidemics in otherwise healthy people, not just in pregnant women or people with immune deficiencies. Additional concerns have been raised by studies linking T. gondii to schizophrenia, obsessive-compulsive disorder, rheumatoid arthritis, brain cancer, and even to kids' trouble in school.

"The accumulation of Toxoplasma gondii oocysts, found in cat feces, may be a much bigger problem than we realize because of their apparent long life and their association with some diseases," said E. Fuller Torrey, who directs the Stanley Medical Research Institute.

He calls for better control of the cat population, especially feral cats, and more research. Surveys have shown that our backyards and communities may harbor three to 400 oocysts per square foot or more in places where cats frequently leave deposits. Each and every one of those oocysts has the potential to cause an infection.

As for the cats, they typically become infected upon hunting and eating an infected bird, mouse, or other small mammal. Then, they spread oocysts around into the soil, grass, water, and elsewhere.

For cat owners, there is little need to worry if your cats stay indoors, Torrey says. If your feline friend (or your neighbors') does spend time outside, take care with litter boxes, keep sandboxes covered, and wear gloves when gardening. One estimate shows that the dirt under ones fingernails could harbor up to 100 T. gondii oocysts.

Torrey and coauthor Robert Yolken of Johns Hopkins University Medical Center recommend extra care with young children, who may be at the greatest risk. But, at this point, there are still many unknowns.

Is it worth getting tested? "No," Torrey says, except perhaps in the case of pregnant women. "Fifteen percent of us have antibodies, including me." And, he adds, someone who tests positive at one point in time can later test negative.