herd immunity

     I've had people tell me that they don't need to be vaccinated for contagious diseases because they have "herd immunity". By this they mean that since other people are vaccinated, they won't get sick, even without the vaccination. Probably not so, and quite risky. If most people are vaccinated, a disease might be checked before it gets anywhere in a population--but then again, it might not. Herd immunity belongs to a herd. If most people are vaccinated for a deadly disease, most people will survive an outbreak of it, since only the unvaccinated will become ill and die. The herd will remain, undamaged for the lack of the few people who weren't vaccinated. Individuals don't have herd immunity. Herds do.