Republican presidential candidates Ben Carson, Donald Trump and Rand Paul responded to a question about vaccines and autism at the GOP debate. (CNN)

It was possibly the easiest question of the night when Ben Carson, a pediatric neurosurgeon, was lofted a no-brainer about whether Donald Trump should stop repeating one of the most debunked, fear-mongering ideas in medicine: that vaccines cause autism.

But instead of introducing a rare moment of fact-based clarity, with a simple and declarative answer, the question opened a huge can of worms and a meandering, speculative, and vague discussion of vaccines that left physicians and public health officials around the country cringing last night.

Carson did undermine Trump's claim about the connection between vaccinations and autism. But he then veered into a very murky area, raising questions about whether children should be getting vaccines that aren't for deadly diseases.

"Vaccines are very important. Certain ones. The ones that would prevent death or crippling," Carson said. "There are others, there are a multitude of vaccines which probably don't fit in that category, and there should be some discretion in those cases."

Across the country, doctors shouted at their TV's, "which ones is he talking about?"

"He entered this netherworld: 'it seems like they’re getting too much and obviously there were the diseases that can really kill you and the ones that don't,'" said Paul Offit, a professor of pediatrics at Children's Hospital of Philadelphia. His opponents, Offit said, should have asked for a list of the supposedly not deadly diseases that could safely be taken off the list.

[The fraudulent origins of the idea vaccines cause autism.]

Here's the truth: there are vaccines for 14 different diseases given in the first few years of a child's life, according to a carefully vetted schedule. These may be for diseases, like measles and diphtheria, that we generally don't think of as killers today -- but that's largely because vaccines have been so successful in preventing people from getting sick in the first place.

Those vaccines are scheduled so that they can be given to children before they come into contact with the pathogens that cause disease. When they are given in combinations, or "bunched" at the same time, it's only after they are carefully tested in "concomitant use" studies to make sure the vaccines don't interfere with each other or cause harm.

The most true thing anyone said about vaccines all night might have been when the candidates aired their own discomfort, as adults, at seeing a small child getting a shot. Trump described the distress of seeing a baby of getting a shot that looks like it's for a horse. Rand Paul said that his kids got their vaccines, but said  he was concerned about how they were bunched up.

"Even if the science doesn't say bunching them up is a problem, I ought to have the right to spread my vaccines out a little bit at the very least," Paul said.

But parents have to get their kids to do lots of things that aren't pleasant and yet are good for them. Indeed, experts say, when doctors stray from the bunching of vaccines, they fall into unknown territory where the harms and benefits are less clearly understood.

"It’s not like the CDC makes it up. They give these vaccines in combination only when proven to be safe and effective," Offit said. "When you choose what Ben Carson or Trump or Rand Paul is arguing for, you’re making up a schedule. You don’t know whether that’s safe or effective."

Offit has studied whether children's immune systems might become overwhelmed or weakened by vaccines, and has found that it isn't the number of vaccines that matters, but the number of triggering immunological components -- called antigens -- that those vaccines contain that determine the immune burden. The vaccine for smallpox, the largest mammalian virus, actually has more antigens than all the vaccines given today, due to improvements in medicine and technology. Which means that, paradoxically, despite giving children far more vaccines, we're actually overwhelming children's immune system far less.

Perhaps the ultimate irony came when Mike Huckabee tried to deflect the autism and vaccine argument to a real health care issue - the cost of care. But then he suggested that the government declare war on major diseases like cancer to find cures. Richard Nixon already did that in 1971, and we're still looking for cures today. Meanwhile, a growing number of scientists have come around to the idea that we're going to have to find ways to detect cancers early and even prevent them entirely if we really hope to make a major dent in disease. And that's not just true for cancer.

Alzheimer's disease, another affliction Huckabee wants to declare war on, has increasingly been shifting to earlier interventions. Drug trials are increasingly being tested on people with mild cognitive impairment, after efforts to use drugs to reverse the full-blown illness have failed. Perhaps by the end we'll be treating people before they show signs of illness at all.

Kind of like a vaccine.