I found this interesting article that says :

"..Biologists have long used the degree of 'tooth wear' in koalas to assess how long they have left to live, but its relationship to koala sex habits had never been noted before. Koala teeth wear down as a result of their constant grinding of eucalyptus leaves, their primary source of food.

The blunter their teeth, the less food they can digest. Consequently, most koalas don't actually die of disease or old age - they die of starvation."