A parental prerogative to sometimes prioritize the interests of everyone's children is not enough to defeat a duty to show concern for the vulnerable. This chapter focuses on two kinds of vulnerability. First, some people are especially vulnerable to infection, because they are unable to develop individual immunity through vaccination. Second, some people, particularly, women and the poor, are especially vulnerable to negative consequences of mass disease outbreaks. If the popularity of The Vaccine Book is any indication, many people agree with Dr. Bob Sears about morality of vaccine refusal. Public health officials certainly regret that some people suffer vaccine complications, but they may try to justify these harms by claiming they are more than offset by the great benefits that individual and herd immunity provide. When they lack access to vaccines, their failure to become vaccinated cannot be characterized as free-riding, since under such conditions they lack a means by which to make a fair contribution to herd immunity.