vaccines
vaccines are an interesting mathematical and social problem. vaccines both do good and cause harm. the optimal strategy for an individual is for everyone *else* to be immunized. as more people choose this strategy, the pool of potential victims grows. which increases the chance of an out of control outbreak. which means more people will get vaccinated. eventually you hit a sort of equilibrium. unfortunately, it's at the point where epidemics will occur. "fortunately" is a charged word to use here. but i'm going to use it anyway. from the perspective of those who can afford to be immunized, it's fortunate that disease will only spread among those who are too ignorant or too vulnerable to be vaccinated. the fear is they mutate it into something no one's immune to. we're selecting for people who are vulnerable to vaccines and immune to disease. which is a strategy. a scary strategy.