to identify the healthy humans
scientists struggled to find a way to stop the robots from attacking unaffected humans as well as the zombies. it took a few trial runs, some deaths, and plenty of failures, before they found one. they built tiny robots called nanomachines to be injected into the blood streams of healthy humans.
the robots were calibrated to identify the nanomachines and know not to attack. it would have worked too, if the injections had gone as planned. they were shipped all over the world, but most were never injected. the rich, or the privileged who lived in the fancy bunkers or knew the right people got their injections but few other people. the common people blamed the scientists.