Your body system has something called the major histocompatibility complex. The MHC collects proteins from parasites and shows them to the immune system to start a counterattack. If everybody had the same MHC, a virus that figured out how to evade it would be able to attack with impunity. So the MHC genes are under tremendous selective pressure to to evolve variety, and there are a huge number of mutants out there in the population.

Consider a genetic variation that if you inherit one copy you communicate well but if you inherit two copies you stutter. Communicating well will be selected for, but stuttering will be selected against. If the probability of having one copy is P, then the probability of having two copies is P squared. For example, if 5% of people have one copy, then 0.25% will have two. The variation will be selected for until it is so common that the gain in fitness for one-copy people is balanced by the loss in fitness two-copy people. (The real situation is more complicated, since variations in many genes interact to affect each other's fitness.)