It doesn't take much for an entire friend group to catch the same sickness -- especially if that sickness is mono, and you're all inclined to make out with each other. But as it turns out, there are a few good things that are also highly contagious: A new scientific study found that being pro-social -- doing things for the common good even if when it costs us something personally -- has a ton of viral potential.

The study, published by the Center for Economic and Policy Research, looked at blood donors in Switzerland and found that the roommates, live-in romantic partners and neighbors who live in the same building as people who donated were much more likely to also donate than people who were just randomly asked to do so.

Thanks to an earlier study, the researchers knew going in that people who get a phone call urging them to donate because their specific blood type is in short supply a few days before a blood drive are significantly more likely to donate. They randomly made those calls to a number of people, and then measured the likelihood that the roommates, lovers, and neighbors of the people who got the calls would donate blood, too -- and they found that those people were also significantly more likely to donate blood, even though their own blood type wasn't reported as being in short supply.

The study also found that the motivation within those social networks created a "positive feedback loop" for the original donor, suggesting that the act of being pro-social can spread especially rapidly and effectively within families, tight-knit groups of friends, or the members of clubs or teams.

If you do something good, then your friends do too, you feel good about it -- and so do they, so their friends start doing it too, and the goodness keeps spreading from there, because everybody's feeling great about being such awesome people.

Clearly this means that when you do something brilliantly pro-social and post about it on social media, it doesn't count as Facebragging -- it's actually just spreading motivation and inspiring others to go out and do something good for the world, too.

So when it comes to your good deeds, don't hesitate to, um, "metaphorically" make out with all your friends -- go spread that pro-social contagion.