You’ve heard it: “The best part about getting a vaccine isn’t the lollipop -it’s the part where you don’t get sick and die.” Just ask anyone who remembers the panic of polio or smallpox outbreaks. But nothing in this world is totally risk free. Most medications ignite side effects in some people, and even if the relative number of those people is tiny, science takes note. Kurzgesagt shows us how vaccines work, using measles as the example, and explains the odds of side effects in numbers we can understand. The most common side effect of the MMR vaccine is fewer dead children.