Blood Sisters (Great Science Fiction Stories)

Why waste years designing viruses for biological warfare when blind mutation and natural selection was all that was required? The theory was, they'd set up a few trillion copies of their system. The theory also included 520 people all sticking scrupulously to official procedure, day after day, month after month, without a moment of carelessness, laziness or forgetfulness. Apparently, nobody bothered to compute the probability of that or of finding wonder drugs.