disease
diseases have two basic strategies. there's the highly infectious route. spread as rapidly as possible. kill your host. and jump to reservoir when you run out of hosts. examples would be small pox, bubonic plague, measles, etc. dangerous diseases. because they kill healthy people. we combat them with herd immunity. reduce the number of available hosts to zero. the other strategy is to tax your host enough to spread but not enough to kill it. examples would be cold and flu. this is a tricky business for the disease to strike the perfect balance. this is why there are so many different strains. the ones that get it right survive to next flu season. those too taxing or too nice, don't. these diseases are not deadly to healthy people. usually the weak succumb. these diseases mutant rapidly. this makes them difficult to combat. i'd argue, we don't want to. herds of animals are healthier when there's a predator culling it. humans are the largest herd on earth. we're also the top predator. nothing preys on us. except disease. which puts me in the seemingly odd position of saying get vaccinated against killer diseases. but don't get flu shots.