If enough people are vaccinated against a contagious disease (in the case of measles, 95 percent of the population), they create protection for the unvaccinated. This six-second gif gives a pretty good explanation of how herd immunity works.
When immunity is above 95 percent, a disease is unable to spread easily and is contained to just a few sporadic cases. When immunization levels are below 95 percent, it's an entirely different story.
As a simulation tool from the University of Pittsburgh shows, immunization levels don't have to get very low before outbreaks start to look quite alarming.
The simulation tool, called the Framework for Reconstructing Epidemiological Dynamics (FRED), shows how measles would spread in your city if immunity levels in people under 15 were to fall as low as 80 percent, vs if 95 percent of children under that age were immune.
If you go to their website and enter your home city (or a city you'd like to see hit with simulated measles) you can watch the outbreak play out under both scenarios side by side.
"The simulation begins with a single school-age child contracting measles, and shows the possible spread of the disease in the six months after the initial case," the creators write on their website.
"Red dots show the location of infectious people, and blue dots show the location of recovered people. If more than a few cases appear, herd immunity has been lost, and the disease spreads easily. If only a few cases appear, herd immunity is still in place."
It's quite a visceral way of seeing the importance of getting vaccinated, not just for your own protection, but for the protection of those around you. In both scenarios, it's assumed that people above the age of 15 have 95 percent immunity. The difference that it makes when just 20 percent of people under the age of 15 are unvaccinated is quite dramatic.
People have been trying it out, and are a little alarmed by the difference.
Try it out for yourself here. And if you're still unconvinced about the benefits of vaccination, try to make it the whole way through Roald Dahl's heartbreaking letter to anti-vaxxers without changing your mind.