evidence: The measles outbreak traced back to Disneyland has spread to 14 states, with as many as 102 cases reported. Media outlets are highlighting the rise of anti-vaccination sentiments. Scientists are expressing their dismay at people who reject sound medical advice and put their families and communities in harm’s way. [...] Let’s look at the concept of herd immunity first. The basic idea is that a group (the "herd") can avoid exposure to a disease by ensuring that enough people are immune so that no sustained chains of transmission can be established. This protects an entire population, especially those who are too young or too sick to be vaccinated. But how many people need to be immune to achieve this? [...] More importantly, there is an ethical argument to be made for the goal of 100 percent vaccination coverage. It sends the right message. Everyone who can get vaccinated, should get vaccinated – not only to protect themselves, but to protect those who can’t, through herd immunity.