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Is depression contagious? The short answer is yes, but like most things, the answer is complicated: It's not as if you'll become "infected" when a depressed friend cries on your shoulder. Your susceptibility or immunity depends on a number of things, from your to your history and levels.

It’s been known for almost a decade that both healthy and unhealthy behaviors are contagious—if your friends or become obese, you’re more likely to do so, too. Even can come in clusters.

Depression occurs along with a range of unhealthy attitudes and behaviors, including , criticism, canceling of social plans, and general irritability. And it turns out that these behaviors—and the negative beliefs that drive them—can be communicated from person to person. The roommates of depressed college students, the children of depressed parents, and the spouses of depressed partners may also begin to express depressive symptoms.

And it’s not just the people you see every day who can have this effect on you—symptoms can actually spread within up to three degrees of separation. (If Kevin Bacon becomes depressed, all of Hollywood is going down.)

Consider a 2014 study of college roommates: Researchers studied more than 100 pairs of newly assigned freshman roommates at move-in, and then again three and six months later. They examined, among other things, the students’ symptoms of and their tendency to ruminate—their propensity to get tangled up in their feelings and to obsess about the causes and consequences of not feeling well.

Sure enough, students who lived with a ruminating roommate also developed the tendency, which greatly increased their own risk of depression. To be clear, the depressive symptoms themselves weren’t contagious, but the thinking styles were. After six months, freshmen who “caught” a ruminative way of thinking from their roommates had twice as many depressive symptoms as those who didn’t pick up the thinking style.

A 2015 study showed that depression can be made contagious under laboratory conditions, at least in rats. Researchers induced depression in rats by exposing them to unpredictable, uncontrollable stressors over several weeks—an approximation of chronic stress in people. For the rats, this included things like keeping the lights on for 48 hours straight and spilling water on their bedding. These stressors were enough to depress the rats—for example, they became less interested in sugar water, a lab rat’s greatest pleasure. This is a marker of anhedonia, a hallmark symptom of depression in people as well as, apparently, rats.

After the rats became depressed, the researchers introduced some new roommates so that two depressed rats and one newly introduced, non-depressed rat were housed together. It turns out, they found, that living with another depressed creature is, in fact, depressing, even for rats. Within a few weeks, the new rats exhibited the same symptoms as their depressed peers.

We can’t replicate such a controlled experiment with humans—who would let researchers come to their house and spill water on their bed?—but it makes sense. With time, a negative outlook can become convincing. If your depressed roommate or partner is critical, withdrawn, apathetic, and pessimistic, you may get caught up in the negativity, too.

Does this mean you should spend less time with the depressed people in your life? Only you can answer that, but as you continue to engage with them, take the time to communicate that he or she is loved, that they are important to you, worthy of your love, and deserving of feeling better. Encourage them to seek help, but know that it may take an incredible amount of bravery on their part (and patience on yours) to take that first step.

But also remember: You can’t rescue your loved one by yourself. You’re up against a host of uncontrollable variables and there may come a point when you need to prioritize saving yourself. Depression can annihilate any glint of , making it difficult to find the will to eat, shower, or, most important, seek help.

On a more hopeful note, it’s not only depressive thinking that’s contagious: Positive emotions and thinking styles can be infectious, too. Think of the rush of excitement at a sports event or concert, the palpable calm after a yoga class, the simple courtesy of service with a smile, and of course, the warm fuzzies from hugging someone you love. Indeed, in the same roommate study cited above, freshmen who were paired with a roommate whose thinking style was more positive also “caught” that healthier thinking style.

So we know that emotions are contagious—and while your partner isn’t the only factor, depressive thinking definitely plays a role in whether your connection leads you into despair or helps you stand strong as a support system.

A version of this piece originally appeared on Quick and Dirty Tips.

Disclaimer: All content is strictly for informational purposes only. This content does not substitute for mental health care from a licensed professional.

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