Libby Copeland spells it out:
Vaccinated children are threatened because, according to the CDC, childhood vaccines are effective for most, but not all, of those who receive them. For about 5 to 15 percent of us, the vaccine won’t cause us to develop immunity. And we might not find that out till we’ve contracted the disease. There are also babies too young to be immunized, and seriously ill people, such as those with leukemia, who can’t be. In California in 2010, nine infants too young to be vaccinated died during an outbreak of whooping cough. In other words, when you vaccinate your child, you’re not just doing it for her, or for the sake of other children whose parents might not vaccinate them. You’re doing it for every one of us.