PHILADELPHIA – The apparent source of the new coronavirus, an urban market with a collection of live animals that somehow infected shoppers, may have struck many Americans as something that couldn’t happen here.

But viruses have been spreading from animals to humans for many thousands of years, ever since we began to farm with them, live among them, and kill them for food.

Measles is thought to have descended from a virus that infected cows. Rabies comes from dogs and cats. And influenza originated in birds, though it tends to pass through and evolve in other animals, such as pigs, before acquiring the ability to infect people.

That is what happened with the 2009 swine flu, which killed thousands of people worldwide. It is why Pennsylvania officials took drastic precautions when the flu swept through chicken farms in 1983 and 1984, even though that strain of virus had not gained the ability to infect humans. Farmers were ordered to kill millions of infected birds.

Scientists have yet to pinpoint the source of the new coronavirus, which has infected more than 70,000 people and killed nearly 1,800, most in China. A likely candidate is bats, which are known to harbor other viruses from the same family. Chinese researchers think it may have first passed through an intermediate – a scaly animal called the pangolin that is used in traditional Chinese medicine.

Tracing the family tree of viruses is tricky, as they swap genetic material like kids trading baseball cards, said Sara Cherry, a professor of microbiology at the University of Pennsylvania’s Perelman School of Medicine. What’s clear is that viruses were jumping from one kind of creature to another well before humans came along, likely as far back as the beginning of life itself.

“This has been happening for a long time,” said Tony L. Goldberg, a professor at the University of Wisconsin-Madison School of Veterinary Medicine.

And the consequences are not necessarily bad.

Viruses are a key part of what make us human, said Luis P. Villareal, professor emeritus of molecular biology at the University of California, Irvine. In addition to genes, the human genome contains countless bits and pieces of viral genetic material – remnants from when our ancestors were exposed to various viruses in the past.

These snippets are part of what scientists used to refer to as “junk DNA” – which is not junk at all, but bits of information that have a role in how genes are regulated, Villareal said.

“If you compare our genetic sequences to some of the other primates, this is what really varies,” he said. “This stuff, this parasitic, infectious material, is really the driver of speciation.”

Still, viruses can cause deadly disease, and epidemiologists would dearly like to identify what it takes for them to jump from animal to human, so they can prevent the next epidemic.

It is not an easy task.

In some cases, a virus can jump from species to species with as little as one “point” mutation – a change in just one letter of the virus’s genetic code, Cherry said. That happened in the 1990s, when a rodent virus gained the ability to spread easily between horses, causing a type of equine encephalitis. On another occasion, a seemingly small change in a canine virus led to its spread among raccoons.

“We’re bad at predicting,” Cherry said. “We don’t know what the barriers are.”

A virus might need one kind of genetic change to gain the ability to latch onto cells in the human respiratory system, for example. Some other change might be required so that it becomes adept at making copies of itself inside human cells. Still another mutation might improve its ability to spread from human to human.

Such changes might be more likely in an environment with multiple kinds of animals, allowing for the wholesale swapping of genetic material, said Linda Van Blerkom, a Drew University professor emerita of biological anthropology.

So live animal markets, such as the one in Wuhan thought to be the origin of the new coronavirus outbreak, may increase the risk of viruses spreading from animal to human, she said. But plenty of other human activities do so as well, such as when we clear forests, exposing ourselves to wild animals. Another common source is agriculture – a phenomenon first documented thousand of years ago.

The Greek historian Thucydides described a mysterious plague that killed thousands of Athenians in 430 B.C. as they prepared to fight off invaders from Sparta. Modern researchers have proposed a variety of viruses as the culprit – among them a measles-like virus, as that is thought to have originated in cattle, which humans domesticated during that period, Villareal said.

A few hundred years later, in The Iliad, Homer described a deadly, virus-like pestilence that struck mules and dogs before spreading among the Greek warriors who attacked Troy.

Viruses can jump from animals to humans over and over, most commonly through the bites of mosquitoes and ticks. West Nile virus and dengue – so painful it is nicknamed “breakbone fever” – are transmitted from animal to human by mosquitoes.

It works in the other direction, too. In 2013, dozens of chimpanzees in a Uganda national park developed a severe cough, and five died. Researchers pinpointed the culprit as a type of rhinovirus that normally infects humans, and have since identified two more such human viruses in wild primates.

“Viruses don’t really care what direction they go in,” said Wisconsin’s Goldberg.