Nearly half of Americans embrace some type of medical conspiracy theory, according to a study published this year by the University of Chicago. So it's no surprise that a new crop of outlandish beliefs around the spread of the Ebola virus have started to take hold.

Rush Limbaugh, for example, has suggested that liberal politicians believe that the U.S. is culpable for Ebola patients in Liberia, where some freed American slaves settled in the 19th century, and that belief is keeping them from imposing travel bans. Others, based a bizarre misreading of a patent for a specific strain of the virus filed several years ago by the Centers for Disease Control and Prevention, believe the government is teaming up with the pharmaceutical industry to profit from a possible vaccine. Or if you follow hip-hop artist Chris Brown on Twitter, Ebola is a form of population control. His tweet has been retweeted more than 44,000 times.

Medical conspiracy theories pop up around any widespread health scare, sometimes bolstered by the inadequate or opaque government responses that can follow. Such theories captured the public imagination during the AIDS crisis in the 1980s, surfaced around the 2009 H1N1 influenza outbreak, and they remain a staple of the anti-vaccination movement. Conspiracy theories represent a way for people to try to make sense of a chaotic health threat -- especially one like Ebola that's horrific and far from being contained overseas.

"You're going to have gaps in the signals that are coming in about what's happening in the world, and you're going to want to fill in those gaps somehow," said Jesse Walker, books editor of Reason magazine and author of The United States of Paranoia: A Conspiracy Theory. "If you're afraid of something, you're going to find a fearful pattern. Obviously infectious disease is something people are very afraid of."

Ebola is a terrible, deadly virus that has devastated Western Africa. Health officials say it is not a significant threat in the U.S., but fear remains rampant: Two out of three Americans are concerned about a domestic outbreak, according to a recent Washington Post/ABC News poll. And that fear, coupled with a deep distrust for the federal government and healthcare system, is fertile ground for conspiracy theories.

"It does connect up to people's fear of the federal bureaucracy -- whether it's the FDA with pharmaceuticals, or the federal government and its relationship to the medical complex and insurance companies," said Mark Fenster, a law professor at the University of Florida and author of Conspiracy Theories: Secrecy and Power in American Culture.

The University of Chicago study found that some medical conspiracies tied to the government were especially prevalent. For instance, more than a third of those surveyed believed that the Federal Drug Administration is withholding cures for cancer because of pressure from pharmaceutical companies.

Conspiracy theories currently circulating that speculate the U.S. government is involved in spreading Ebola resemble decades-old theories about HIV and AIDS, Fenster said. Back then, a belief surfaced that the Central Intelligence Agency was using the HIV virus to research germ warfare, alongside a theory that the virus was engineered to wipe out the black and gay populations.

Unfortunately, several shameful, calculated historical precedents help to explain the emergence of theories related to the medical abuse of minorities. The infamous Tuskegee syphilis study did involve conspiratorial behavior by researchers, who failed to inform their black male subjects that the study was examining the effects of untreated syphilis -- over the course of 40 years, from 1932 to 1972.

Eugenic sterilization laws were on the books in most states until the 1970s, resulting in the involuntary sterilization of more than 60,000 Americans. Just this year, North Carolina became the first state to offer monetary compensation to some of the victims of such a policy. Between 1929 and 1974, that state sterilized some 7,600 people, citing reasons that included being lazy or poor. More than 60 percent of the victims were black, according to state records.

"With that soil, it's been easy for people to suggest that in the 1980s, white doctors were infecting black babies with AIDS," said Walker.

Twelve percent of Americans still believe that the CIA knowingly injected large numbers of African-Americans with HIV, according to the University of Chicago study.

Whether conspiracy theories take their inspiration from past bad behavior, free-floating fear or cynical political ploys, the United States is not the only place they crop up.

"All you have to do is look at Africa to see the way in which the fears of western medicine and former colonial powers have affected how they view Ebola," Fenster said.

One theory, perpetuated in a Liberian newspaper, suggests that the U.S. Department of Defense manufactured Ebola as a bioterrorism weapon and is testing it in Africa.