The devils have a low genetic diversity which is consistent with a 'founder effect'. What this means is: a small number colonised Tasmania from Australia at some stage. So their genetic variety was much less than the parent population. Disease like the present may have happened before, and reduced the population to a small number. These events are called 'population bottlenecks'. A small population which carries less variation is always vulnerable to extinction, because none of the animals may be resistant to the infection.