As the shadow of the deadly Ebola virus looms over Africa, its danger clear and present, a lesser kind of contagion, diffuse but persistent, has infected social media feeds in the United States.

Ebola is trending on Twitter. Even a cursory hashtag search turns up, among the news articles and official announcements, expressions of fear, gallows humor and bad information. The virus can spread through the air? OMG! (It cannot.) A possible Ebola case in New York City? Time to pack for Mars! (It was not Ebola.)

Why do people feel compelled to post and rebroadcast jokes, rumors and dread of a distant disease that public health officials say is extremely unlikely to pose serious risk on this side of the Atlantic Ocean?

The science behind how and why ideas spread on social media is a growing area of research. At the most basic level, marketing experts say, people tend to share stories that stir their deepest feelings, whether positive or negative. To wit, frightful shark attacks routinely top the trending charts alongside cheerful cat videos and inspirational quotes.