“It was long thought they were just junk DNA,” said Shawn L. Chavez, a molecular biologist at the Oregon Health and Science University School of Medicine in Portland, who wrote a review of research on endogenous retroviruses in mammals. Now it is clear that some of them have changed the course of evolution. Exactly how is what scientists are trying to find out. “It seems like there’s a new publication every day,” she said.

Consequently, koalas are drawing a lot of attention from scientists who did not start out with an interest in the animal or its conservation. “I’m a fruit fly guy,” Dr. Theurkauf said. He became interested after a report in 2006 by Rachael Tarlinton of the University of Nottingham and other scientists about the invasion of the koala genome by the retrovirus.

Dr. Tarlinton began her career in Australia as a veterinarian with an interest in infectious diseases in wildlife. She became involved in the study of koala genetics because of the problem of chlamydia and because Jon Hanger, an independent researcher, had noticed very high death rates from leukemia and other cancers in koalas kept in zoos. Their research led to the discovery that koala retrovirus was causing some of the cancers and that it was not only infecting the animals but part of their genome.

Dr. Tarlinton and her colleagues established the presence of the retrovirus in koalas in Queensland, but there is another, more southern population of koalas that at first seemed not to have the virus. These koalas also had fewer chlamydia infections. The genetics of the southern population are different because most koalas in that region had been killed for the fur trade by the 1920s. A small number survived by being moved to small islands in the early 20th century.

“From that population, they’ve been reintroduced,” Dr. Tarlinton said. And those koalas have done extraordinarily well, even though they have some genetic problems. There are tens of thousands of them. In some areas they have been killed to keep the population down.

The researchers expected the southern koalas to be less healthy than the northern ones, she said. But the opposite was true.