Loneliness is more than just a feeling, according to an article in the New Republic. It’s a biological process that activates your physical pain responses and trashes your immune system.

Here’s one of many fascinating (and, okay, probably depressing) examples of the very tangible effects of loneliness, from a study of gay men with HIV during the ’80s:

The social experience that most reliably predicted whether an HIV-positive gay man would die quickly, Cole found, was whether or not he was in the closet. Closeted men infected with HIV died an average of two to three years earlier than out men.