Half of the adults in the survey agreed with at least one medical conspiracy theory

Half of all adult Americans believe in at least one medical conspiracy theory, including that the the CIA infected the African American population with HIV.

In a survey led by researchers at the University of Chicago, 1,351 adults were asked whether they had heard of six popular medical conspiracy theories. They were also asked if they believed or disagreed with the theories.

Approximately 18 percent agreed with three or more theories.

The theories also included claims that the US government is aware mobile phones cause cancer yet do nothing about it.

The highest percentage agreed with: "The Food and Drug Administration is deliberately preventing the public from getting natural cures for cancer and other diseases because of pressure from drug companies."

Around 37% of participants agreed with this statement.

Other theories include the CIA infecting large numbers of African Americans with HIV, as well as suggesting there is truth behind a discredited link between vaccinations and autism. Another claims water fluoridation allows companies to flood dangerous chemicals into the environment.

Around 49% of those asked agreed with at least one of the theories, which had distrust of the government or large corporations as a common theme.

Eric Oliver, the lead author of the study, suggested the reason such a large proportion of Americans believe the theories is because they are easier to understand than scientific explanations.

Oliver also added that people who believe in one or more of these theories are more likely to use alternative medicine, instead of conventional treatments.

According to the Huffington Post, believers were less likely to have flu innoculations or use sunscreen.

Oliver told Live Science that psychological conditions such as paranoia are not the reason for such beliefs. Instead, he suggests people have a natural tendency to assume malevolent forces are behind the "unknown".

Oliver commented: "These narratives seem like very compelling explanations for complicated situations."

He added that humans may have evolved to think in such a way: "If you hear a noise in the bush, it's much more adaptive to believe that there's a predator there than not."

In 2012, Oliver compiled a study on a similar topic, entitled Conspiracy Theories, Magical Thinking and the Paranoid Styles of Mass Opinion.

In the paper, Oliver examined four nationally representative surveys taken between 2006 and 2011 that measured public opinion on conspiracy theories.

The paper said that conspiracy theories are encourged by "magical thinking", which he defines as "a propensity to attribute the source of unexplained or extraordinary events to unseen, intentional forces".