Half of Americans believe in medical conspiracy theories. That’s dangerous.

To vaccinate or not to vaccinate. That ... really shouldn’t be the question. But for some reason it is in America.

Deadly diseases such as measles and polio are no longer a threat to the majority of vaccinated Americans. (As The Times’ editorial board recently wrote: “Vaccination doesn’t immunize every person who gets the shots; some of the recent California cases were among people who had been vaccinated.”)

Despite widespread fears, scientific consensus has shown no correlation between the MMR (measles, mumps, rubella) vaccine and autism. A 1998 report in Lancet that started the controversy has since been widely debunked. And yet a University of Chicago survey published Monday in the journal JAMA Internal Medicine reveals that half of Americans believe in medical conspiracy theories. A full 20% believe the government and the medical world are pushing vaccinations on children that cause autism.

That number is both startling and extremely dangerous.

It’s worth going back in history a little here. A similar paranoia to the one we’re seeing gripped Britain over the pertussis vaccine in the 1970s. During that time, after seeing virtually no cases of the disease, 100,000 incidents of pertussis and 36 deaths from the disease were reported. In Japan several years later, after pertussis vaccination levels dropped from 80% to 20%, an outbreak of the disease spread to 13,000 people, causing 41 deaths.

In infants, pertussis can cause pneumonia, seizures and mental retardation, according to the Centers for Disease Control and Prevention. The disease killed upward of 9,000 Americans a year before widespread vaccination began.

Before the polio vaccine, 13,000 to 20,000 Americans were left paralyzed from the disease each year.

Then there’s the inexplicably controversial MMR vaccine. Anti-vaccination activists such as actress Jenny McCarthy and former MTV reality star Kristin Cavallari claim that, in addition to the disproved autism link, vaccination can cause asthma and ear infections.

Even if that were true, it’s important to remember that vaccinations aren’t given for fun. They prevent deadly and crippling diseases.

Mumps causes women to miscarry; rubella often leads to blindness, deafness or mental impairment in infants; and even with modern medical care, as many as 3 out of every 1,000 people who contract measles die from the disease.

In that context, who in their right minds would be worried about ear infections and asthma?

It’s also worth noting that just because Americans largely don’t get these diseases anymore, it doesn’t mean the threat has somehow mysteriously vanished. A 2011 outbreak of measles in Ethiopia, a country without a robust vaccination regime, infected at least 17,584 people, killing 114.

Incidentally, despite reporting challenges in a poor and rugged country, estimates show that autism rates in Ethiopia are around 1 in 100 — comparable to the 1 in 88 we see in America. Despite the lack of widespread vaccinations, the disease is still epidemic.

2013 was a banner year for measles in the United States. A disease that had all but vanished within our borders suddenly infected 189 people. A recent outbreak in New York infected 20 people.

We in America have the ability to prevent diseases that kill millions around the world. And yet our ignorance is allowing these diseases to make a comeback. So if you’re worried about vaccines giving your children ear infections or asthma, try taking a look at what measles does to a child. It isn’t pretty.

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Matthew Fleischer is a Los Angeles-based freelance journalist. Follow him on Twitter @MatteFleischer.