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When I talk to parents who are hesitant about vaccines, what they most want to talk to me about are possible side effects of the vaccine. They worry about everything from fevers and soreness to additives to possible links to autism. They rarely worry about the diseases that vaccines prevent—and that’s what worries me most of all.

It is the inconvenient truth of vaccine refusal: when you don’t get vaccinated against an illness, you are more likely to catch it.

A study published in the Journal of the American Medical Association (JAMA) made this very clear. Researchers looked at information about recent measles and pertussis outbreaks. They found that unvaccinated people made up the majority of those who caught measles and a large proportion of those who caught pertussis (waning immunity from the pertussis vaccine plays a role in those outbreaks). Some weren’t old enough to be vaccinated—but of those who were old enough, most came from families who had chosen not to vaccinate.

We developed vaccines for a reason: to stop children from getting sick and dying. This was not a money-making stunt by drug companies, as some claim. Here in the United States, vaccines have done such a great job that we have literally forgotten about the ravages of measles, polio, pertussis, diphtheria, and the many other illnesses that we can now prevent.

They truly were ravages. Who even remembers diphtheria? Between 1936 and 1945, there were about 21,000 cases and 1,800 deaths a year of diphtheria. In those same years, paralytic polio affected 16,000 and killed 1,900 each year. And for measles and pertussis, the numbers are even higher. Every year 530,000 people caught measles and 440 people died from it; 200,000 caught pertussis and 4,000 died from it.

Four thousand died every year from pertussis. In 2014, that number was 13. We simply cannot ignore that vaccines are incredibly effective, and save thousands and thousands of lives.

It’s the scarcity of the illnesses that has made some parents comfortable with the decision not to vaccinate. If you are unlikely to run into anyone with measles or chickenpox, why take any chances with side effects?

There are two problems with that argument. First, as more people have chosen not to vaccinate, there have been more outbreaks. And when those who choose not to vaccinate live in the same communities, as a study out of Kaiser Permanente showed was the case in California, it can create the perfect environment for a vaccine-preventable germ to spread.

Second, we live in a global community. Travel is relatively easy, and lots of people do it. And while we may have done a great job eradicating vaccine-preventable diseases here in the U.S., they certainly haven’t been eradicated from the world.

People are often contagious before they even know they are sick. Someone could bring measles to a community without knowing it—and 90% of the unvaccinated people who are exposed to the measles virus will get sick (the virus can even hang out in a room for two hours after the person with measles leaves). Half of the babies who catch pertussis end up hospitalized—and of those who are hospitalized, three out of five have trouble breathing, and one in 100 die despite the best possible care.

We just can’t say that it’s safe to be unvaccinated. It’s not safe for the child whose parents choose not to vaccinate—and it’s really not safe for the infants or people with immune problems who cannot be vaccinated, who need vaccinated people around them to keep them well.

Vaccines are a medical treatment, and like any medical treatment, they can have risks and side effects. So much has been done, and is still being done, to make vaccines as safe as possible. It’s always important to ask questions and be careful in making decisions.

But when making those decisions, it’s crucial to think not just about the vaccine—but about the disease it can protect you from.