That's in stark contrast to HIV, which took years to identify as the cause of AIDS.

But many of the tools used by public health teams to halt outbreaks are the same they've used for centuries: quarantine, public education, and good hygiene.

Still, "Public health has always responded to crisis," Osterholm says. "The luxury is that crises tend to occur now and then. Today, we're in a constant state of crisis. If you tell your firemen they have to be on call 24/7, they get pretty fatigued."

One way to help is for ordinary people to be on guard, he says. For instance, we should all be more aware of the hazards posed by our pets.

In 1975, the U.S. government banned miniature pet turtles when the CDC reported that the reptiles were the source of 14 percent of all human salmonella infections in the country. The salmonella bacteria is carried by most reptile species and can lead to infections like sepsis and meningitis in people. More than 93,000 cases of pet-related salmonellosis are reported each year, according to the CDC. Most cases are mild, but in 1997, an eight-year-old boy died after handling pet iguanas -- the animal, after miniature turtles, that may be the riskiest.

"When we see children hospitalized with salmonella meningitis," the most severe form of the bacteria, says Osterholm, "you can almost certainly predict there are iguanas in the household or a neighbor's house."

A 2001 study found that 13 percent of American pets are infected with a parasite that causes the disease giardiases. The bug is generally picked up by pets in drinking unsafe water, but it can be transferred to people if -- ick -- they accidentally ingest particles of the pet's feces.

Ferrets have been popular pets in the U.S. for decades, but they can carry rabies if not vaccinated. Photo: CDC.

Rabies, a virus that infects the human brain and nervous system, remains a worldwide threat. Although actual infections are rare, the disease is serious enough that any mammalian pet should be vaccinated each year, says the CDC. Dog bites are the most common cause of rabies, but ferrets can also carry the virus. (Incidentally, rabies may be re-emerging in South America as mining and logging operations push mammals out of Amazon forests.)

Dogs can also transmit a bacterium known to pet boarders and breeders as "kennel cough." People can catch the bug from particles in the air, but only sometimes get sick. Still, if your dog comes back from a kennel hacking like mad, a visit to the vet may avert one to the family doctor.

Although cat bites account for only five percent of all animal bites in people, they are more likely than any other kind to spawn infections. Cat-scratch disease (AKA "cat scratch fever"), is caused by a bacterium that is harmless to cats but can cause flu-like symptoms in people who have been bitten or scratched. Cats also carry toxoplasmosis, a parasite that can infect people who come into contact with the contaminated feces. It can be especially dangerous for pregnant women, sometimes causing spontaneous abortion.

Very young puppies and kittens are susceptible to hookworms and roundworms. Again, people can be infected if they accidentally swallow particles from feces -- where the worms' eggs are often found. Yet another reason to scour away those pawprints tracking across your floor. Thankfully, worms are easily spotted and treated by veterinarians.

Birds harbor a bacterium in the Chlamydia family, that causes the respiratory infection psittacosis in people. The CDC reported more than 800 cases of the illness in people over ten years, more than 70 percent of which were associated with birds. Symptoms are flu-like, but the disease can be serious if left untreated. It is also notoriously difficult to diagnose in people.

Rats, hamsters, gerbils and other rodents carry a range of zoonoses, including a bacteria that causes "rat-bite fever." The disease is rarely serious, but it can cause flu-like symptoms and a prominent rash. These species -- especially guinea pigs -- can also carry salmonella.

Flying squirrels carry a bacterium that causes cause typhus. They can also harbor plague.

Don't get too close. Flying squirrels like this one may be cute, but they can carry typhus. Photo: USGS

And, although we hate to say it, even rabbits can be dangerous. Domestic rabbits rarely bite and are free of most transmittable diseases. But they can carry tularemia, a very infectious bacterial disease. Two cases have just emerged in Kansas, says Osterholm, after a man mowed over a family of rabbits in his lawn. The bacteria spread to the air, and both the mower and one of his neighbors have contracted the disease.

We might keep in mind, too, that humans can also spread disease to other animals, says Hahn. "We're awfully human centric when we talk about these diseases," she says. But ecotourism in Africa, for example, has led to a lethal disease called cryptosporidiosis in mountain gorillas.

And when two species that are ordinarily separated by oceans or mountain ranges (like, say, a prairie dog and a Gambian giant rat) get together, they can infect each other, too. SARS and monkeypox have reminded us that pathogens often heed no borders -- be they geographic or biological. And sometimes, we unwittingly chauffeur them around.

-- Sarah Goforth

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