THE spread of Ebola from western Africa to suburban Texas has brought with it another strain of contagion: conspiracy theories.

The outbreak began in September, when The Daily Observer, a Liberian newspaper, published an article alleging that the virus was not what it seemed — a medical disaster — but rather a bioweapon designed by the United States military to depopulate the planet. Not long after, accusations appeared online contending that the federal Centers for Disease Control and Prevention had patented the virus and was poised to make a fortune from a new vaccine it had created with the pharmaceutical industry. There were even reports that the New World Order, that classic conspiracy bugbear involving global elites, had engineered Ebola in order to impose quarantines, travel bans and eventually martial law.

While most of these theories have so far lingered on the fringes of the Internet, a few stubborn cases have crept into the mainstream. In the last few weeks, conservative figures like Rush Limbaugh and Laura Ingraham have floated the idea that President Obama had sent aid to Africa, risking American lives, because of his guilt over slavery and colonialism. And just days ago, the hip-hop artist Chris Brown took to Twitter, announcing to his 13 million followers: “I don’t know ... but I think this Ebola epidemic is a form of population control.”

Conspiracy theories have always moved in tandem with the news, offering shadow explanations for distressing or perplexing events. Though typically dismissed as a destructive mix of mendacity and nonsense, they often reflect societal fears.