You don't understand OP, diseases don't generally try to kill (90+% mortality rate) the host, really the only diseases that try are diseases from large animals that jumped the barrier, this is much less likely to happen, lyme disease has been traced to mice, a small animal, so much less likely to kill humans when they get it than smallpox which went from gerbils (not very lethal) to cows (not very lethal) to humans (very lethal)
