Why did myxomatosis become less effective at controlling rabbits in Australia?

Myxomatosis became less effective at controlling rabbit populations in Australia due to the coevolutionary process between the myxoma virus and the European rabbit.12

When the virus was first introduced in the 1950s, it was highly virulent, killing nearly 99 percent of infected rabbits within two weeks. However, as the rabbit population declined, natural selection began to favor mutant strains of the virus that were less lethal. These strains allowed the host to survive longer, which increased the virus's opportunity to spread to other rabbits before the current host died.12

Simultaneously, the rabbit population underwent selection for increased resistance to the infection. This ongoing coevolution resulted in an interaction that is less immediately lethal, with most infected rabbits now surviving for two and a half to four weeks rather than succumbing as quickly as they did during the initial outbreak. Consequently, the virus no longer provides the same level of population control it once did.21