They detected a wide diversity of strains and, for purposes of analysis, they grouped those strains into five distinct grades of Australian myxoma, on a descending scale of lethality. Grade I was the original strain, with its case fatality rate of nearly 100 percent; grade II killed upward of 95 percent; grade III, the intermediate among all five, still killed between 70 and 95 percent of infected rabbits. Grade IV was milder, and grade V milder still (though far from harmless), killing less than 50 percent of the rabbits it infected.
