It used to be thought that chlamydia psittaci caused disease in koalas but new research indicates that this is not so. However, research into the disease continues and our knowledge is constantly being updated. Scientists now believe that the chlamydia organism has been endemic amongst koala populations for many years, and has acted as a natural population control in times of stress. The weaker animals succumb to the disease, become sick, infertile or die, leaving the genetically stronger animals to continue breeding. However, this is not the case in most mainland populations, and indeed many of the mainland colonies are in decline.
Koalas also suffer from a range of cancers like leukemia and skin cancers.