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TOORBUL, Australia – 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. “We can observe what the vaccine does under real conditions,” said Peter Timms, a microbiologist at the University of Sunshine Coast in Queensland who has spent the past decade developing a chlamydia vaccine for koalas.

In koalas, chlamydia’s ravages are extreme, leading to severe inflammation, massive cysts and scarring of the reproductive tract. 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.

Because of these similarities, the vaccine trials that Timms and Endeavour Veterinary Ecology, a wildlife consulting company, are running in wild kaoalas may offer valuable clues for researchers who are developing a human vaccine.

Chlamydia is the most common sexually transmitted infection worldwide, with 131 million new cases reported each year. In the U.S., 1 in 10 sexually active teenagers is infected, said Dr. Toni Darville, chief of the division of pediatric infectious diseases at the University of North Carolina.

Antibiotics exist, but they are not enough to solve the problem, Darville said. That’s because chlamydia is a “stealth organism,” producing few symptoms and often going undetected for years.

In 2019, Darville and her colleagues received a multiyear, $10.7 million grant from the National Institute of Allergy and Infectious Diseases to develop a vaccine. The ideal package would combine a chlamydia and gonorrhea vaccine with the HPV vaccine already given to most preteenagers.

Chlamydia’s stealth and ubiquity — the name means “cloaklike mantle” — owes to its two-stage life cycle. It starts out as an elementary body, a sporelike structure that sneaks into cells and hides from the body’s immune system. Once inside, it wraps itself in a membrane envelope, hijacks the host cell’s machinery and starts pumping out copies of itself. These copies either burst out of the cell or are released into the bloodstream to continue their journey.

“Chlamydia is pretty unique in that regard,” said Ken Beagley, a professor of immunology at Queensland University of Technology and a former colleague of Timms. “It’s evolved to survive incredibly well in a particular niche, it doesn’t kill its host, and the damage it causes occurs over quite a long time.”

The bacterium can hang out in the genital tract for months or years, wreaking reproductive havoc. Scarring and chronic inflammation can lead to infertility, ectopic pregnancy or pelvic inflammatory disease. Evidence also is mounting that chlamydia harms male fertility as well.

No one knows how or when koalas first got chlamydia. But the curse is at least centuries old. In the late 19th century, Australian naturalist Ellis Troughton noted that the “quaint and lovable koala” was also susceptible to disease.

The animals suffered from an eye ailment similar to pink eye, which he blamed for waves of koala die-offs in the 1890s and 1900s. At the same time, anatomist J.P. Hill found that koalas from Queensland and New South Wales often had ovaries and uteruses riddled with cysts. Many modern scientists now believe those koalas were probably afflicted with the same scourge: chlamydia.

Koalas today have even more to worry about. Dogs, careless drivers and, recently, rampant bush fires have driven their numbers down so far that conservation groups are calling for koalas to be listed as endangered. But chlamydia still reigns supreme: In parts of Queensland, the heart of the epidemic, the disease helped fuel an 80% decline over two decades.

The disease is also the one that most often sends koalas to the Australia Zoo Wildlife Hospital, the country’s busiest wildlife hospital. “The figures are 40% chlamydia, 30% cars, 10% dogs,” said Dr. Rosemary Booth, the hospital’s director. “And then the rest is an interesting assortment of what trouble you can get into when you have a small brain and your habitat’s been fragmented.”

Booth’s team treats “chlamydia koalas” with an amped-up regimen of the same antibiotics used on humans. “I get all of my chlamydia information from the CDC,” she said, referring to the Centers for Disease Control and Prevention, “because America is the great center for chlamydia.”

But the cure can be as deadly as the disease. Deep inside a koala’s intestines, an army of bacteria helps the animal subsist off eucalyptus, a plant toxic to every other animal. “These are the ultimate example of an animal that’s completely dependent on a population of bacteria,” Booth said. Antibiotics extinguish that crucial gut flora, leaving a koala unable to gain nutrients from its food.

For the past decade, Timms has worked to perfect a vaccine. His formula, developed with Beagley, appears to work well: Trials have shown that it is safe to use and takes effect within 60 days and that animals show immune responses that span their entire reproductive lives. The next step is optimizing it for use in the field.

Wasn’t it unusual to have an animal that gets such humanlike diseases: diabetes, cancer and sexually transmitted infections? “We are but an animal,” Booth said, throwing her hands up in a gesture of unity with the world. “We didn’t think of it first.”