As a general rule, pets are good things to have.

Studies have shown that watching fish in an aquarium can lower blood pressure. People with heart disease who are pet owners tend to survive longer than those who are petless, another study found. Pet visits enliven the day for nursing home residents and have been used to combat stress and depression among teenage inmates Rikers Island in New York.

But there's a down side.

Tick bites, flea bites, cat scratches, even the water from an aquarium can cause an assortment of ills that are revolting at best and occasionally even fatal.

Most of the ills transmitted by pets to humans can be either prevented or treated. But diagnoses may be missed because people don't always inform doctors that they have a pet.

Other than rabies, most pet owners are unaware of any specific diseases that animals can transmit to humans; many are infections that are easily prevented and may be ignored until they develop into something serious.

An article by two Georgetown University professors published recently in the American Family Physician, the journal of the American Academy of Family Physicians, is designed to alert doctors to the kinds of unpleasant diseases that are borne by pets.

Everybody knows about rabies, from the bite of a "mad dog" or other infected animal. An acute viral infection that attacks the nervous system, rabies, known to the ancient Greeks, was once invariably fatal.

Rabies, which infects raccoons and skunks as well as cats and dogs, is still lethal if treatment is not administered promptly after the bite. Although publicity, vaccination and prompt treatment have reduced the prevalence of rabies in the U.S. to little more than a case every two years or so, there are more cases and deaths in developing countries, according to Peter M. Schantz, an epidemiologist with the parasitic control division of the federal Centers for Disease Control.

Diseases such as Rocky Mountain spotted fever -- now seen all over the country -- and Lyme disease are frequently spread by ticks carried in by pets.

Although these diseases are the second most prevalent animal-spread illness, they lag far behind bites and scratches that may transmit ailments to millions every year, according to Schantz.

Both dogs and cats carry natural germs in their mouths that can cause illnesses in people. "Most of the public health attention focuses on the hind end of the pet," Schantz says, "but it really is the front end that causes most of the trouble."

Some lesser-known ills, transmitted from four-legged, gilled or winged family members, can sometimes be serious and occasionally fatal.

Georgetown physicians Jane H. Chretien and Vincent F. Garagusi note in their article that the 40 million dogs, 30 million cats and "millions of rodents, birds, reptiles and tropical fish" who inhabit about 60 percent of American homes make the potential for exposure to pet-related diseases "enormous." The family dog may even be a strep carrier; and people can contract poison ivy by petting a cat or dog who's been rolling in it.

Chretien, medical director of the Georgetown University Student Health Service, says she got interested in the subject because so many college students were coming in with infections they were picking up from pets.

In healthy adults, many of the problems may cause only mild flu-like symptoms. But for young children, and in the case of one particular infection, pregnant women, results can be devastating.

Here are some of the common infections and suggestions from Schantz and Chretien about how to prevent them: Bites and scratches. Schantz says that up to 2 million people a year actually require medical treatment for dog bites; 20 people die from attacks by dogs every year. Even nips from dogs and cats can cause flu-like illnesses, such as cat scratch fever and an assortment of bacterial infections. Cat bites, because of the animal's needle-like teeth, are more likely to become infected.

Prevention: Never leave a toddler alone with an animal. Treat a bite as you would any dirty wound. Check to make sure the animal has had a rabies vaccination and see that the person gets a tetanus shot, if not recently vaccinated. Intestinal roundworm. This parasite is intestinal only in the animals. In humans, it migrates through tissues. In very small children, the parasite can end up in the eye where, if left untreated, it can cause blindness. Schantz says there are about 1,000 such eye cases annually in the U.S., which may mean that tens of thousands more are infected in other ways.

The parasite is transmitted through the feces of puppies or kittens. Eggs contaminate the area and may be accidentally ingested by toddlers who, Schantz notes, "taste everything."

Prevention: New puppies should receive anti-worm medicine as early as two or three weeks of age. Maintain meticulous hygiene for young children who play around animals. Toxoplasmosis. Almost half of the people in the U.S. are infected with this organism, which is transmitted in cat feces and is commonly present in inadequately cooked meat. In most cases, the hundreds of thousands of cases go unnoticed, but a primary infection in a pregnant woman can cause "devastating" damage to the developing fetus, says Chretien.

Among the consequences of toxoplasmosis are miscarriage or a multitude of birth defects, including retardation. Schantz says that about 3,000 babies are born with congenital toxoplasmosis every year.

Prevention: A pregnant woman should avoid uncooked meat and should never, never clean cat litter boxes. Ringworm, scabies. Ringworm is a fungus, not a worm, and scabies is caused by a mite. Both cause world-class itching but are usually not dangerous.

Prevention: A veterinarian can often spot these in the animals. New pets should always be checked by a vet. "Swimming pool" granuloma. This comes from fish tanks, generally not from well-chlorinated swimming pools. It is caused by a marine organism and sometimes affects fishermen. Chretien says the only case she has seen was in a tropical-fish researcher at Georgetown. Psittacosis, also known as parrot fever and nontyphoidal Salmonella, is associated with pet birds and those cute little pet turtles they used to sell in dime stores. Stringent regulations on the interstate sale of both birds and turtles have reduced the incidence of these ailments.

Prevention: Forget turtles. Be sure birds come from reputable dealers.

In general, says Schantz, "when you consider the prevalence of pet ownership, people get an incredible amount of exposure, yet disease is pretty rare. The most important thing is that people are totally unaware of these diseases, so they are not inclined to take the simple precautions that they'd be glad to take if they knew."

Schantz, who is scheduled to speak to a group of veterinarians this week, believes they should be more active in telling people about potential health problems from pets. "But," he speculates, "they don't like to emphasize the down side."