Rachel Gross of the New York Times writes, the first sign is the smell: smoky, like a campfire, with a hint of urine. The second is the koala’s rear end: If it is damp and inflamed, with streaks of brown, you know the animal is in trouble. Jo, lying curled and unconscious on the examination table, had both.

Jo is a wild koala under the purview of Endeavour Veterinary Ecology, a wildlife consulting company that specializes in bringing sick koala populations back from the brink of disease. Vets noticed on their last two field visits that she was sporting “a suspect bum,” as the veterinarian Pip McKay put it. So they brought her and her 1-year-old joey into the main veterinary clinic, which sits in a remote forest clearing in Toorbul, north of Brisbane, for a full health check.

Ms. McKay already had an inkling of what the trouble might be. “Looking at her, she probably has chlamydia,” she said.

Humans don’t have a monopoly on sexually transmitted infections. Oysters get herpes, rabbits get syphilis, dolphins get genital warts. But chlamydia — a pared-down, single-celled bacterium that acts like a virus — has been especially successful, infecting everything from frogs to fish to parakeets. You might say chlamydia connects us all.

This shared susceptibility has led some scientists to argue that studying, and saving, koalas may be the key to developing a long-lasting cure for humans. “They’re out there, they’ve got chlamydia, and we can give them a vaccine, we can observe what the vaccine does under real conditions,” said Peter Timms, a microbiologist at the University of Sunshine Coast in Queensland. He has spent the past decade developing a chlamydia vaccine for koalas, and is now conducting trials on wild koalas, in the hopes that his formula will soon be ready for wider release. “We can do something in koalas you could never do in humans,” Dr. Timms said.

In koalas, chlamydia’s ravages are extreme, leading to severe inflammation, massive cysts and scarring of the reproductive tract. In the worst cases, animals are left yelping in pain when they urinate, and they develop the telltale smell. But the bacteria responsible is still remarkably similar to the human one, thanks to chlamydia’s tiny, highly conserved genome: It has just 900 active genes, far fewer than most infectious bacteria.