When the disease reached epidemic proportions the humans became their own worst enemies. The pandemic struck at the worst possible time: artificial intelligences had escaped from a lab in the guise of perfectly replicated human bodies.
It was impossible to tell at even a modest distance who was flesh and blood and who carbon-steel and hydraulic fluid.
Luckily, these androids had no reason to fear or hate humanity; they simply wanted to explore. That was in their programming. But paranoia would not be dulled, and under the tensions of the break-out, the outbreak added a new variable, as well as a new tool. Someone soon realised that the bots would be immune to the virus.
But, humanity being what it is, a terrible ‘test’ emerged, whereby suspected droids were kidnapped and exposed to the pathogen. If they lived, they were immediately destroyed. If they were human, the death was slower but no less gruesome.