I have written a book that explains why we use vaccines to prevent disease. I have also written a book about mental illnesses: what they are and how they are classified, diagnosed, and treated. So I am well prepared to answer a common question: are the antivaccine zealots out of their minds? The answer is “sometimes”; that determination must be made on a case-by-case basis. Vaccination has long been the mainstay of public health efforts. Being vehemently opposed to the mainstay of public health efforts is not, per se, a mental illness. But neither is it a sign of good mental health, especially not in an educated person. In fact, people who go to great lengths in their antivaccine advocacy are more likely than the average person to have a particular kind of mental illness: a personality disorder.