The measles outbreak at Disneyland in California is one of the largest the United States has experienced in years, affecting at least 67 people in five states. But this fact, from USA Today, is arguably the scariest part of the public health emergency:

Six of the cases were in infants too young to have been vaccinated.

The measles vaccine is not licensed for use on babies younger than 12 months. That means that, for the first year of life, babies depend on the fact that everybody else around them gets vaccinated. This essentially creates a firewall: if other people are vaccinated, they won't catch the disease — and won't spread it to young children who cannot get protection.

This is what scientists call "herd immunity," and its a huge reason we get vaccines in the first place. The shots aren't just about protecting ourselves from measles, mumps, the flu, or other diseases. They're about making it really hard for those who are medically frail (like the elderly) and those who can't get the vaccine (often babies and pregnant women) to catch a disease that could be devastating to them. The vaccinated people form something like a fence around the vulnerable people, making it extra hard for the disease to come in.

At Disneyland, the fence wasn't high enough — and it didn't do a good enough job protecting babies against measles because there were too many people who didn't get vaccinated.

As my colleague Julia Belluz has explained, this isn't just about Americans who refuse to get vaccinated, but also foreign travelers coming from countries dealing with outbreaks. In both cases, measles is the most dangerous to those who can't get protected against it.