Parents who decline vaccines may not realize that they're gambling with the lives of not just their kids, but all the children around them, says Patsy Stinchfield, director of pediatric infectious disease at Children's Hospitals and Clinics of Minnesota, where Landon was treated. Measles can kill by causing pneumonia, brain inflammation and other complications, Stinchfield says. Babies too young to be vaccinated and people with compromised immune systems, such as those with cancer, are especially vulnerable. They rely on others around them to keep the virus out of circulation, a phenomenon known as "herd immunity," which protects even those who can't be vaccinated, she says.
Story here. The article notes that the incidence of measles, while still low in an absolute sense, has seen a spike this year. There have been, so far, 152 diagnosed cases of measles this year. Suprisingly, Minnesota leads the way with 23 documented cases followed by New York (21 cases) and California and Massachusetts (17 each).
In my Micro Principles class, I use vaccines against infectious diseases as examples of products with a positive externality. The quoted paragraph above makes it clear why: when a person gets a vaccine against, say, measles, not only does he lower the chance he'll get the measles but he lowers the chance that someone else will get the measles
When a product has such an externality, there is an efficiency argument to be made for government intervention in the market for the product. The intervention can take many forms, including the subsidization of the production/consumption of the product, requirements to consume the product, and government provision of the product. What the most efficient form is depends on the particular situation.
HT to Christopher Tozzo for the link.