Ever hang with a friend who brought you down with her bad mood or get caught up in a giddy moment with someone being silly? This phenomenon is called emotional contagion (EC).

Exactly as it sounds, EC means that other people's emotions are contagious. Even more interesting, one study found that you don't have to be face-to-face with that person to "catch" the mood; you can do on it Facebook or any social media site. A study looked at people who had reduced emotional content on their news feeds and found that when positive expressions were reduced, people produced fewer positive posts and more negative ones; when negative emotions were reduced, the opposite occurred.

Results indicate that emotions expressed by others on Facebook influence our own emotions.

We already know that babies and moms share more than just a bond. They transfer their emotions to one another. A study in Psychological Science found that a mom's stressful experiences are contagious to her infant and that members of close pairs, like mothers and infants, can reciprocally influence each other's physiological reactivity. In other words, their emotions are contagious to one another.

What emotions are most contagious?

It may seem strange, but there are studies into which emotion gets shared most — and anger tops the list. Scientists at Beihang University in China evaluated millions of messages posted to a micro-blogging site similar to Twitter and found that anger spreads most quickly.

Users seek not only to express their anger but instill a sense of anger in others in their online communities, in essence helping them to catch the outrage.

And in yet another study from the University of Pennsylvania, the most popular contagious viral emotion was awe. The study author in his book, "Contagious: Why Things Catch On," writes that science articles have a high probability of spreading because they "frequently chronicle innovations and discoveries" that evoke a feeling of awe.

What can we tell about a person from social media posts?
Developed by researchers, the Analyze Words website uses celebrities' Twitter posts to reveal the main emotions they portray. It found that Kim Kardashian's mood was mostly upbeat while Taylor Swift's was depressed.

But it's not just anger that you can catch online. A study in the British Medical Journal found that the spread of happiness reaches up to three degrees of separation. Though person-to-person effects are strongest, the effects decay well before reaching the whole network. But the reach of a particular behavior or mood spiral is not limitless, and can even effect friends of friends. So if you post exciting, happy news, not only do your friends become happy too, but their friends experience happiness as well. Research suggests this "three degree of influence rule" applies to depression, anxiety, loneliness, drinking, eating, exercise and many other health-related activities and emotional states.

How can all this help you?

While catching somebody's happiness doesn't seem like such a bad thing, you should steel yourself from catching your social network's negative moods. Here's how to protect yourself:

  • Keep your own mood in check. Keep your facial expression neutral or happy even if you don't feel like it. Research shows your mood will follow your expression.
  • Focus on spreading positive emotional contagion. Make eye contact in person, share good news online, and use humor.
  • Inoculate yourself against negative EC. Simply being aware that moods are contagious can help you from "catching" a bad one. Limit your exposure to chronically negative, sad or depressed postings online.
  • Pay attention to your own online language. Emotion is expressed through tone, words, language, inference — even emoticons. Keep yours positive so you don't cause others unnecessary negativity.