The koala, one of Australia’s most treasured creatures, is in trouble.

Faced with habitat loss, climate change and bacterial disease, koalas are being pushed into smaller and smaller regions of the country. In Queensland, the vast state in Australia’s northeastern corner, surveys suggest that from 2001 to 2008, their numbers dropped as much as 45 percent in urban areas and 15 percent in bushland.

And while climate change and habitat loss are affecting many other uniquely Australian animals, too — from birds and frogs to marsupials like wombats, wallabies and bandicoots — it is a bacterial infection that is worrying many scientists about the fate of the koala.

“Disease is a somewhat silent killer and has the very real potential to finish koala populations in Queensland,” said Dr. Amber Gillett, a veterinarian at the Australia Zoo Wildlife Hospital in Beerwah, Queensland.

The killer is chlamydia, a class of bacteria far better known for causing venereal disease in humans than for devastating koala populations. Recent surveys in Queensland show that chlamydia has caused symptoms in up to 50 percent of the state’s wild koalas, with probably even more infected but not showing symptoms.