They cuddle and purr. And they shed. They wag their tails and fetch your slippers. And they shed. They never talk back and they never hold a grudge. And they shed.

There are obvious pluses and minuses to living with pets, not only with respect to your happiness and housekeeping, but also with respect to your health. Here's a sampling of some of what scientists know about how pets can affect your physiological and psychological well-being — the good, bad and downright gruesome.

THE GOOD

Health: An Australian survey found that dog and cat owners were in better health than people with neither (health was measured either by how often people went to the doctor or by how much medication they took). And a study with people on Medicare found that those who owned pets made fewer doctor visits than those who didn't.

Hypertension: A number of studies have found that just being around a dog or petting a dog can lower blood pressure. One study found the same with a pet goat. Another found that simply watching a Lassie movie was enough to lower stress.

Longevity: A year after being released from a coronary care unit, a 1980 study found, pet owners were more likely to have survived than people who didn't have pets.

Bone strength: The sound frequencies ofcats' purrs are between 25 and 150 Hertz. Some researchers have found that sound frequencies between 20 and 50 Hertz can improve bone density and speed the healing of bones and muscles. So maybe that purr … don't laugh. Some scientists actually have suggested this.

Allergy prevention: Evidence is mounting that children raised with pets are less likely to develop allergies to the animals than children raised without. In at least one study, the effect was greater with cats than withdogs. And in at least one other, the preventive effect extended to dust mite, ragweed and grass allergies.

Obesity: A study in Australia found that children who had a dog in their household were less likely to be overweight or obese than children who didn't.

Fitness: In one study, two out of three dog owners took Fido for regular walks. Younger owners were more likely to walk than older owners, and younger dogs were more likely to get taken out than older ones. Bigger dogs got to go on longer walks than smaller ones. Another study found that dog owners were 60% more likely to go for walks in their leisure time than people who owned cats or who didn't own any pet. Finally, a third study suggests that if you want to shape up, dogs make better walking buddies than humans do — perhaps because dogs don't make up excuses for why they can't go that day.

Smoking: Almost 30% of pet owners who smoked said they'd try to quit if they were convinced that secondhand smoke could hurt their pets, a survey found. (Less than 2% said the same thing about their children.)

Schoolwork: Several studies have reported that young children who had had pets (goldfish, hamsters or dogs) were better at making simple biological inferences than children who had never had a pet. Another found that students in a 10-week reading program who practiced reading out loud to dogs improved their skills by 12%. The students in the program who didn't read to dogs didn't improve at all.

Math: Pet owners who had lower blood pressure than non-owners to begin with experienced less of a rise in that pressure when they had to do mental arithmetic. Blood pressure rose least of all for those owners whose pets were with them while they made their calculations.

Heroism: A pit bull who saved a baby from a burning house was recently in the news. Many animals, and especially mammals, are hard-wired to save their own babies from danger, says Pluis Davern, a professional dog trainer in Gilroy, Calif. "But the fact that this dog has encompassed a human baby in its sense of family is probably uniquely canine."

THE BAD

Cost: Whether you rescue a stray kitten off the street or you spend a few thou for a Westminster-bound chow, the cost of acquiring a pet is a drop in the water bowl compared to the cost of taking good care of it through its life. The American Society for the Prevention of Cruelty to Animals has calculated how much you can expect to fork over every year to keep your pet in fine fettle: From $875 for a large dog down to $35 for a fish. These are minimums, the association warns: "You should definitely be prepared to pay more."

Allergies: If you're allergic to dogs or cats, and there's one in your vicinity, you're likely to cough and wheeze and sneeze. Your eyes are likely to itch and your nose is likely to run. And you're not likely to enjoy it all very much.

Aggression: About 4.5 million Americans are bitten by dogs every year, with 1 in 5 bites causing injuries that require medical attention. Children ages 5 to 9 are most likely to be injured, and children are more likely than adults to need medical help.

Those claws: If you get scratched (or bitten) by a cat, you could get cat scratch disease, but only if the cat has fleas, says Bruno Chomel, a UC Davis professor of zoonoses (diseases that can be passed from animals to humans). Flea feces are the source of the problem, he says, and the cat may give it to you if it has some under its claws. Symptoms include a bump or blister where the scratch (or bite) occurred, fatigue, fever, headache, a swollen lymph node and just a general all-around crummy feeling.

Cat litter: Toxoplasmosis is a parasitic disease that can be transmitted through the feces of infected cats. Cats can only get infected if they hunt and eat rodents or if their owners feed them raw meat. And as a rule, even an infected cat can only transmit the disease for two weeks in its entire life. It's rare for people to get toxoplasmosis from their cats, but the disease is very dangerous for pregnant women because it can lead to birth defects or miscarriages. Have someone else clean your cat's litter box, or wear rubber gloves while you do it, and wash your hands very well after you're done. Change the litter every day (the protozoa in cat feces can't infect anyone until the feces are at least a day old). Still, remember: The most common way for people to get toxoplasmosis is by eating undercooked meat.

Sickness: You can catch quite a bit of unpleasant stuff from your pets — including ringworm, MRSA and the plague. And people with compromised immune systems are at extra risk. Still, while pets can carry and pass along a variety of parasites, you can greatly reduce any danger just by washing your hands after petting or playing with them, according to the Centers for Disease Control and Prevention. On the other hand, studies have shown that sleeping with your pets increases your risk. "I would not say that having your dog on the bed is bad," Chomel says. "But it's better to have it by the side of the bed. And having it under the covers is definitely going too far."

Don't even read this: Last month a woman in Van Nuys woke up to find that part of her big toe was missing. The woman had numbness in her foot caused by diabetes, so she didn't feel a thing during the night when her 2-year-old Jack Russell terrier started chewing it. (The toe was infected and had an open sore, which would have been a natural attraction to many dogs.) After surgery to amputate part of her injured toe, the woman developed other infections and ulcerations, and eventually doctors were forced to amputate the lower part of her leg.

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