Researchers believe this kind of social isolation is not only making us physically ill, it’s changing the ways in which we seek companionship. Loneliness is not simply introversion. It isn’t the same as preferring to be alone. Rather, it’s a gap between the amount of social interaction a person would like to have and the amount they experience.

We can even see this loneliness gap in other species, like rhesus monkeys. Normally, scientists determine loneliness by giving people questionnaires about how they feel. That doesn’t work so well on monkeys, which are otherwise a fitting animal model for human loneliness, so John Capitanio, a research psychologist at the University of California, Davis, had to devise a new method. He observes the monkeys and divides their socializing into “attempts” and “successes.” For example, the monkey might walk by another monkey in the hopes that it will give it a signal that it wants to interact. “Lip-smack me, or touch me!” the monkey seems to say. If the “attempt” is successful, the monkeys might move on to actual socializing, by, say, grooming one another.

The lonely monkeys make a lot of these attempts at socializing, but they have trouble converting them into true social experiences. Not unlike lonely humans, they are forever asking other monkeys to the prom and getting shot down.

Capitanio sees loneliness as being an instinct almost like hunger. Evolutionarily, it could be our body’s way of telling us that we must urgently go out and seek connection. But that’s easier said than done: lonely humans have an overactive sense of social threat—a fear that they’ll be rejected if they try to reach out and socialize. Lonely people want to be around others, but they are afraid that if they try, they will be rejected.

The monkeys’ brains closely resemble our own, and Capitanio has found that loneliness affects humans and monkeys in similar ways. The lonely monkeys’ urine contains more norepinephrine, which suggests their “fight or flight” instinct is more highly and frequently activated. What’s more, the bodies of lonely monkeys (and lonely humans) tend to have more of a certain kind of white blood cell called monocytes, suggesting that they are in an inflammatory state. And, in an especially strange twist, the lonely monkeys also have higher levels of cortisol, an anti-inflammatory molecule.

“Those should not be happening together,” Capitanio says, of the monocyte and cortisol combination.

Here’s what that combination would look like, using an above-the-skin example. Let’s say you brush up against poison ivy and develop a rash—that’s inflammation. It’s your body’s way of trying to contain the offending substance. You would go to the drugstore and buy hydrocortisone anti-itch cream. Hydrocortisone is a type of glucocorticoid, similar to cortisol.