The invader

In the early days of the settlement of Australia, enterprising settlers unwisely introduced the European rabbit. This rabbit had no natural enemies in the Antipodes*, so it multiplied incredibly quickly. It overran a whole continent, and caused devastation to herbage which might have maintained millions of sheep and cattle. Scientists discovered that this particular variety of rabbit was susceptible to a fatal virus disease. Trying to create local epidemics of this disease, they found out that there was a type of mosquito which acted as the carrier of this disease and passed it on to the rabbits. So while the rest of the world was trying to get rid of mosquitoes, Australia was encouraging this one. It effectively spread the disease all over the continent and drastically diminished the rabbit population.