Do vaccines cause autism?

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An Iraqi child is vaccinated by a nurse during an immunization program in Baghdad. See more modern medicine pictures.

Your health is one of those things you don't think about too much unless something's wrong. People who are sick think about their health a lot. People who have sick loved ones think about health a lot, too. And for the past several years, some people whose loved ones have autism have thought about vaccines a lot.

Vaccines aren't exactly a modern invention. After realizing that people who survived smallpox never got it again, people started inoculating themselves around 200 B.C., hoping that a little bit of exposure in the present would save them from a devastating illness in the future [source: National Museum of American History]. Unfortunately, primitive methods, which used a live virus from infected material, could result in the patient getting a full-blown case of the disease.

The big idea behind modern vaccines is this: You introduce a weakened (attenuated) or dead form of a disease into your body so that your immune system learns how to make the antibodies that fight it. That way, if you ever run into the real thing, your body is prepared to clobber it.

And vaccines work. They may be the single greatest medical discovery in history. Your body can take care of some sicknesses, but other diseases race through your system like wildfire and destroy your body. Smallpox, the Great Influenza, the Black Death -- these epidemics devastated the world's populations, sickening and killing millions of people.

­­Most people start getting vaccinated as infants against a veritable "lions, tigers and bears, oh my" list of diseases: Hepatitis B, rotavirus, diphtheria, tetanus, pertussis, Hib, polio, measles, mumps. If someone asked you to describe the symptoms of all of those diseases, you probably wouldn't be able to unless you're in the medical profession. That's because we rarely see these diseases anymore, because we're protected by vaccines.

­But there's another disorder we're seeing more and more of: autism. Or rather, we're seeing more of a group of conditions, known as autism spectrum disorders (ASDs). Like any disorder, autism is tough. It's especially tough on caretakers. Some kids with ASDs like Asperger's are able to function in society, although with difficulty acting socially. And some kids are barely able to function at all.

There is a group of people who believe that vaccines are causing ASDs. In this article, we're going to investigate whether or not this hypothesis is valid.