Everyone Is vulnerable to stress A male koala receives a swab test at the Sydney Wildlife World in Australia to detect an often-fatal disease called chlamydiosis. Chlamydiosis is caused by a virus that often breaks out in koalas when their immune systems are weakened during times of stress, such as when they are forced to find a new habitat. Fewer than 100,000 koalas are now left in Australia, down from millions of them two centuries ago.