Image caption Koalas love eucalyptus leaves - and they help reassure Penny as she wakes from her anaesthetic

The koala is unique to Australia and is an important symbol of the country. But numbers are plummeting and the survival of koalas is under threat. One of the reasons is the sexually transmitted disease chlamydia.

One of the most common places to find koalas in Australia these days is in the hospital.

About 50 miles (80km) north of Brisbane, at the Australia Zoo Wildlife Hospital, a female koala is under a mild anaesthetic.

"She's quite an old girl - I think she's over 10 years," says veterinary surgeon Amber Gillett.

The koala is called Penny. Gillett puts some ultrasound gel in Penny's pouch and looks at the ultrasound machine.

She's checking her bladder for symptoms of chlamydia.

Outside the clinic are a series of open-air enclosures. These are the "koala wards" - and they are pretty much full all-year round.

Last year, Gillett and her team treated about 300 koalas for chlamydia - and so far, 2013 has been a busy year too.

In people, chlamydia is a common sexually transmitted disease. A different strain infects koalas, but it too can be spread sexually, and it's causing a devastating epidemic. In some parts of Australia, koala infection rates are as high as 90%.

Chlamydia affects male and female koalas, and even the little ones called joeys - who pick it up suckling from their mothers in the pouch.

It causes blindness and infertility in koalas - and can be fatal. Visible signs of infection include conjunctivitis, and a condition dubbed "dirty tail", caused by urinary tract infections and incontinence.

Some other animals are also infected with the disease but it is usually at low levels. It affects koalas more seriously and experts don't know why that is.

Image caption Baby koalas spend the first six or so months in their mother's pouch

But, as in humans, the disease is treatable with antibiotics. This means keeping the koala in captivity for the duration of the treatment - usually a few months - before releasing it back into the wild.

And that's where Jon Hanger, a wildlife biologist, takes over. In a eucalyptus wood outside Brisbane, he unfurls an antenna.

A few facts about koalas A newborn koala is just 2cm long - it continues to grow in its mother's pouch

Koalas spend most of their time sleeping

They eat eucalyptus leaves - not a nutritious diet

Their brains are very small relative to their size

A healthy koala lives about 12-16 years

There were once millions in Australia, but they were hunted heavily for their fur in the early 1900s More koala facts Koalas on BBC Nature

Some of the animals here have been fitted with radio collars, and Hanger follows the signal up to the base of a tall tree.

About 20 ft (6m) up, clinging on to a fork in the tree, is a koala called Maggie.

A couple of years ago, Maggie ended up in the hospital. Her uterus was infected - she had chlamydia - but the vets caught it early and she was given treatment.

"She's had two joeys since then, and hopefully, she's got a third one in the pouch as we speak, so she's a real success story," says Hanger.

But it's not always a happy ending for the koalas. Many have to be put down, and about half of females are left infertile - which means fewer koalas are being born.

Image caption Jon Hanger comes to this woodland regularly to check on the koalas

Experts say it's hard to predict the total number of koalas in Australia, but it's likely to be somewhere between 50,000 and 100,000 - and in some areas, numbers have dropped by as much as 80% in the last 10 years.

Last year the Australian government listed koalas in Queensland, New South Wales and the Australian Capital Territory as a threatened species, and some are even warning the animal could face extinction if more is not done to protect it.

It's not just chlamydia that's threatening koalas. Many are struck by cars, or attacked by dogs. Others are pushed off their land due to urban sprawl.

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And another threat comes in the form of a koala retrovirus, which - much like HIV in humans - suppresses a koala's immune system. It is also having a devastating impact.

Paul Young, a virologist at the University of Queensland in Brisbane says every koala he has tested in the northern part of the species' range has been infected with the retrovirus. And, he says, the "invading" virus is gradually making its way south.

Initially the infection was probably transmitted from koala to koala through physical contact, says Young, - but within the last century, the virus has managed to insert itself directly into koala sperm and eggs.

Because of the retrovirus, many koalas are dying of leukaemia and lymphoma. And it may also be compounding the impact of chlamydia - turning what would otherwise be a relatively minor disease, into something much more serious.

Many young koalas are left orphaned, but there's a small, and committed, army of volunteers who look after them until they are old enough to fend for themselves. They call themselves "koala carers".

"I'm passionately obsessed, I won't apologise," says Wanda Grabowski, who has hand-raised about 40 orphaned koalas in her home since 1999.

Image caption A vaccine for chlamydia in koalas is in the pipeline

"I love everything about koalas - from their little furry ears down to the little poo that pops out of their bottoms!"

But it's not easy rearing these baby animals in their first year of life. "Joeys are most active at night-time, so you have to turn your day into night," she says.

And then, when they're a little older, you have to cut and collect fresh eucalyptus leaf - which can mean driving hours every day.

If all goes well, the koalas are eventually released back into the wild.

Chlamydia in humans A very common sexually transmitted infection

Affects both men and women

Sometimes called the "silent infection" because often there are no obvious symptoms BBC Science on chlamydia

"I always investigate the sites where my joeys go," says Grabowski. "You wouldn't want to send your kid off to some strange place, would you? You'd check it out. So that's what I do as a good mum."

But long-term success in battling these diseases may only come if koalas can be prevented from getting sick in the first place.

Scientists at Queensland University of Technology (QUT) and The Australian Museum recently discovered what they call the "holy grail" to understanding the immune system of koalas - the IFN-g gene.

This should help them work out how, and why, koalas respond the way they do to retrovirus and chlamydia.

Image caption Peter Timms is eager to get his vaccine in use in the field

And they hope to start field trials on a vaccine for koala chlamydia very soon.

The vaccine has not been perfected, says Peter Timms, the microbiologist at QUT who developed it - but at least it's a start. And if it can begin to save lives now, so much the better.

Indeed koalas may have a thing or two to teach us. If the koala vaccine is successful, says Timms, it will act as a model for the development of a human chlamydia vaccine too.

Ari Daniel Shapiro was reporting for The World and the PBS programme NOVA. The World is a co-production of the BBC World Service, PRI and WGBH

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