Significance When a pathogen emerges in a host population, will it evolve to do more or less harm to its host? A single strain of myxoma virus was released as a biocontrol agent against Australian rabbit populations in 1950. The subsequent coevolution has become a textbook classic, although there has been little experimental work on this topic since the early 1980s. Here, we show that the host–pathogen arms race continued with the evolution of highly lethal viruses that cause immune collapse. The possibility that pathogens can become highly immunosuppressive in response to increases in host resistance needs to be considered where genetic and immunologic manipulations are used to enhance host resistance, as, for instance, in agriculture.