Almost half of all American adults believe one or more medical conspiracy theories, according to a study published in JAMA Internal Medicine.

Researchers, led by J. Eric Oliver of the University of Chicago, collected online survey responses from more than 1,300 adults. Oliver and his team asked participants whether they knew of and/or agreed with several common healthcare-related conspiracy theories, including whether:

Regulators restrict access to natural cures

There is a connection between childhood vaccinations and autism

Intelligence agencies deliberately infected African-Americans with HIV

Carcinogenic elements are in cell phones

Corporations dump dangerous chemicals under the guise of water fluoridation

Forty-nine percent of respondents agreed with at least one theory, according to the study. Some were better known than others. For example, 69 percent of respondents were familiar with the vaccination theory, although a plurality of 44 percent disagreed compared with 20 percent who agreed.

The conspiracy theory about natural cures was more popular, according to the study. Thirty-seven percent agreed, compared to less than one in three who disagreed. The HIV theory was both the least popular and the only theory with more than 50 percent disagreement.

"Science in general--medicine in particular--is complicated and cognitively challenging because you have to carry around a lot of uncertainty," Oliver told Reuters. "To talk about epidemiology and probability theories is difficult to understand as opposed to 'if you put this substance in your body, it's going to be bad.'"

Belief in medical conspiracy theories also correlates with how patients approach their own health, according to the study. For example, 35 percent of respondents who believed at least three of the theories took herbal supplements, compared to 13 percent of those who did not believe any conspiracy theory.

"Although it is common to disparage adherents of conspiracy theories as a delusional fringe of paranoid cranks, our data suggest that medical conspiracy theories are widely known, broadly endorsed and highly predictive of many common health behaviors," the study concludes.

Vaccination theories in particular may affect public health. Largely unvaccinated communities were hit hard by three measles outbreaks last year, FierceVaccines previously reported.

To learn more:

- here's the study abstract

- here's the article