In 1859 a wealthy Australian grazier named Thomas Austin imported for sport thirteen wild English rabbits to his estate near Geelong, in Victoria. The rabbits did what rabbits do, and within three years 14,253 of them had been shot on Austin's land. By 1869 more than two million had been killed on a neighbor's property. Soon hundreds of millions of rabbits formed what became known as a "gray blanket" across the continent, destroying native plants, competing with native animals for food and shelter, and savaging grazing lands. In 1950 the government agreed to wage bio-warfare against them, and scientists released myxomatosis, a rabbit-specific pox virus from South America, into the wild. The virus quickly killed 99 percent of the country's rabbits. During the next three years, however, the kill rate among the initial survivors and their descendants dropped to 95 percent; it continued to decline until, eventually, it leveled off at about 50 percent. "It was a classic example of the co-evolution of virus and host," Frank Fenner told me recently. Fenner, a virologist at the John Curtin School of Medical Research, in Canberra, headed the studies analyzing why myxomatosis became less effective. In essence, he said, "you've got this arms race" in which the virus becomes weaker and the rabbit more resistant.
In 1988 a young virologist named Ron Jackson began working at what would later be called the Pest Animal Control division of the Cooperative Research Centre, in Canberra. His goal was to devise a solution that would sidestep those evolutionary forces and work indefinitely. Specifically, he hoped to produce a genetically altered virus that would sterilize rabbits. Jackson initially planned to use myxomatosis, but he couldn't easily get the rabbit genes he needed to engineer the virus. So he switched to mice and a virus called mousepox, intending to perform a "proof-of-concept" experiment that would allow him subsequently to proceed with rabbits. When the project showed early signs of success, he realized that the strategy might also be applied to mice, which bedevil Australia almost as much as rabbits do.