According to a study by two social scientists from the University of Chicago, about half of Americans endorse a medical conspiracy.

An article in the Los Angeles Times noted, "Fully 37 percent of those surveyed endorsed the belief that the FDA, under pressure from pharmaceutical companies, is suppressing natural cures for cancer and other diseases, and 31 percent said they 'neither agree nor disagree' with that idea, the researchers found.... One in five of those surveyed said they agreed that physicians and the government 'still want to vaccinate children even though they know these vaccines cause autism and other psychological disorders.'"

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Many claim that HIV doesn't cause AIDS. Another widely-held conspiracy claims that AIDS was created by the CIA as a way to kill African Americans through vaccination programs. In this study, 12 percent endorsed that belief, while 37 percent neither agreed nor disagreed; only 51 percent of respondents rejected that conspiracy theory.

Filmmaker Spike Lee told Rolling Stone magazine in 1992, "I'm convinced AIDS is a government-engineered disease… they never realized it couldn't just be contained to the groups it was intended to wipe out." Actor Will Smith, in a 1998 interview in the same magazine, shared Lee's belief: "AIDS was created as a result of biological warfare testing. … Someone was messing around in a laboratory, trying to find biological weapons, and created AIDS." (More information on these and other medical conspiracy theories can be found in folklorist Patricia Turner's book "I Heard It Through the Grapevine: Rumor in African-American Culture.")

Though conspiracy theories come in many varieties, those involving medicine and health are among the most prevalent. In his "Encyclopedia of Urban Legends," folklorist Jan Harold Brunvand notes that "Unwarranted fears about poisonous or infectious materials are reflected in a number of legends.

For example the claim that aluminum in cookware or in some antiperspirants can cause Alzheimer's disease or another major consequence has been rejected by scientists as a 'case of neuromythology.'... Serious diseases like AIDS and SARS, but especially the Ebola virus, have been the subject of rumors and legends."

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Fueling Conspiracies

Why such widespread belief in medicine-based conspiracy theories?

For one thing, peddling medical conspiracy theories can be enormously profitable. The highest-profile example is probably Kevin Trudeau, a pitchman best known for appearing in infomercials selling conspiracy-laden best-sellers like "Natural Cures 'They' Don’t Want You to Know About."

Trudeau made millions claiming to reveal important medical and dietary information kept secret by a conspiracy between the medical establishment and big drug companies. Earlier this week Trudeau was sentenced to 10 years in prison; upon sentencing, the judge called Trudeau, who had a previous conviction for fraud, "deceitful to the very core."

With so much money to be made promoting these medical conspiracies by Trudeau and other authors and talk show hosts, it's not surprising that they're so widespread. As conspiracy theory promoters are fond of saying, "Follow the money."

The Internet has also been instrumental in spreading conspiracies. Seth Kalichman, a professor at the University of Connecticut Center for HIV, writes in his book "Denying AIDS: Conspiracy Theories, Pseudoscience, and Human Tragedy," that "AIDS denialists share much in common with 9/11 Truth groups and those who doubt that man ever landed on the moon. All conspiracy theories link a corrupt government with big business to hoax the American public."

Kalichman blames the Internet for the spread of the medical conspiracies: "The Internet has made pseudoscience as accessible, or perhaps even more accessible, than quality medical science. The most easily accessed pseudoscience is delivered by AIDS denialist journalists who often write commentaries for various online magazines, as well as maintain their own websites. Even South African president Thabo Mbeki is said to have solidified his HIV/AIDS denialist views by accessing and ultimately buying into denialist websites."

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A new study has found that half of Americans endorse at least one medicine-based conspiracy theory. iStockPhoto

Another reason that many accept medical conspiracies is that they're plausible and seem to have a grain of truth to them. Suspicions about motives of governments and big business are legitimate and healthy.

The government does not always work in the best interest of its citizens, and huge industries ("Big Oil," "Big Pharma," etc.) are indeed motivated by making a profit (as was conspiracy peddler and convicted felon Kevin Trudeau). The U.S. government has done unethical and illegal things ranging from biomedical research to unwarranted surveillance.

It is undeniable that some doctors and some medical corporations have in the past acted unethically. Examples include shameful events like the infamous Tuskegee Syphilis Study, in which hundreds of poor, illiterate black men were denied treatment for syphilis by government agencies in a medical study that spanned decades, and recently-revealed American syphilis experiments conducted in Guatemala in the late 1940s.

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Thus, for many people it seems like a small leap from genuine wrongs committed by those acting on behalf of governments and powerful corporations to endorsing conspiracy theories. But it's actually a big leap, and that leap requires evidence, not just speculation and theories.

Some may laugh off these beliefs as silly or harmless, but in some cases conspiracy theories have done very real damage to innocent people: For example in the past few years dozens of polio vaccination workers in Pakistan were attacked and murdered because of conspiracy rumors that they were intentionally spreading disease or even trying to abduct children for their organs.