Similarly, Jonah Berger and Katherine Milkman, professors at the Wharton School at the University of Pennsylvania, have found that uplifting articles are more likely than disheartening ones to land on the most-emailed list at The New York Times. But even stories evoking rage or other negative, strong emotions are emailed by readers more often than ones that are simply depressing.

“People share things they have strong emotional reactions to, especially strong positive reactions,” Dr. Guadagno said.

Sharing is not just how information ripples across communities; it’s also how emotions are disseminated. Recently, analysts at Facebook, Yale and the University of California, San Diego, reviewed more than a billion posts by Facebook users, and found that when users vented on a rainy day, their friends in other cities posted bleak status updates more frequently than normal.

But positive status updates were even more contagious, prompting upbeat updates from friends at even greater rates. The conclusion: Online, as in real life, feelings can be caught like the flu.

The most shared post at Upworthy, a site for shareable content, is a video about a boy who died of cancer, but not before producing a hit song and performing sold-out shows. The post has racked up nearly 20 million views, thanks in part to the type of methodically calculated headline that has become the site’s trademark: “This Amazing Kid Got to Enjoy 19 Awesome Years on This Planet. What He Left Behind Is Wondtacular.”