Does this mean that everyone who suffers from heart disease or depression should run to the local shelter and adopt a pet?

No, say the researchers. The positive health aspects of living with a pet seem to work best in people who like animals. "You can't simply prescribe a dog to everybody," explains Hart. "People come along with a history of loving animals -- perhaps certain kinds of animals -- that brings with them those warm feelings. The benefits are there because you have the expectation of these warm, good feelings."

To prove her point she relates the story of the late Aline Kidd, a psychology professor at Mills College who was instrumental in introducing animals in nursing homes, a practice that is widespread today. In one particular facility, she brought in cats and dogs only to find the residents were not interested. They didn't really care if she brought the animals or not. "Then one day, she brought in a pig, and they all loved it. It turned out they'd all grown up on farms with pigs."

Does that mean that if you have fond childhood memories of Snowball the cat, you'll improve your health by adopting a pet now?

Not necessarily. People with allergies to dogs or cats are not encouraged to keep them as house pets. And a committee of the Institute of Medicine reported in January 2000 that cats and dogs, like dust mites, fungi, molds, and cold viruses, can aggravate asthma. The committee recommended the removal of pets from the homes of people with asthma.

Fleas and ticks carried by cats and dogs spread diseases ranging from the annoying itch of a flea bite to Lyme disease. A spokesman for the Society for the Prevention of Cruelty to Animals in San Francisco said his organization urges pregnant women to stay away from cat feces, a possible source of toxoplasmosis, which can cause miscarriage or stillbirth. Reptiles, such as turtles, snakes, and lizards, can spread Salmonella, and families with young children are not encouraged to adopt these as pets. Birds, too, can transmit disease.

In most situations, however, normal precautions -- such as hand-washing after playing with a pet and controlling fleas and ticks, both of which can spread disease -- ensure minimal health risks. Even in immune-compromised AIDS patients Siegel says that the health benefits outweighed the risks if owners adopted safe pet-handling practices.