The pop star posted photos of his infected eye on Instagram.

He may not have intended to, but Justin Bieber is delivering a major PSA on pink eye. The singer posted a close-up photo of one eye, which looked slightly pink, with the caption “This eye has conjunctivitis.” (In fact, this was a series of five photos he posted.)

Considering that just one of those photos scored more than 2.5 million likes, it’s no wonder “conjunctivitis” is now trending. But what does it really mean?

Conjunctivitis is the medical term for “pink eye,” which simply refers to an irritation of the white part of your eye (called the conjunctiva), making it inflamed and red. The condition has three main causes: bacteria, a viral infection, or an allergic reaction to smoke, pollen, or animal dander.

If you have conjunctivitis, your eye might feel gritty, like there’s dirt lodged in it that won't come out. Your eye may itch or burn, and you might see yellowish pus coming out of your eye. And of course, the telltale pinkish hue that colors the whites of your eyes is a tip-off as well.

While conjunctivitis is generally harmless, it's annoying to deal with. And anyone who’s ever been through an outbreak when they were in grade school (or who have little kids right now), knows how scary hearing that someone has pink eye can be, since it has a reputation of being easily transmittable. But not all types of conjunctivitis are contagious, says Rahul Khurana, clinical spokesperson for the American Academy of Ophthalmology.

Both bacterial and viral conjunctivitis can be spread from one person to the next. You can catch pink eye from someone by using an infected towel, touching your eyes, or sharing eye makeup with someone else who had the infection.

“The thought of pink eye often sets off a panic,” says Khurana. “When the conjunctiva starts getting red, everyone can see it,” he says. He notes that three percent of all ER visits are eye-related, and 30 percent of those are for conjunctivitis.

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If you suspect you're coming down with conjunctivitis, you don’t have to head to the ER. But if your eyes look red or pink, see an ophthalmologist who can determine if it’s viral, bacteria, or allergic in nature. Not only does that inform treatment (for instance, a viral infection has to run its course; a bacterial infection may require antibiotic eye drops), but you’ll know if you’re saddled with a contagious form.

If you are, you’ll want to consider staying home from work until symptoms subside. Toss any old makeup you used while you had the infection. Wash your hands after you touch your eye—or you risk passing it on to another person in your household (or even to your other eye). If the gritty feeling is driving you nuts, pressing a cool, wet washcloth over your eyes can make you more comfortable.

Generally, conjunctivitis takes about a week to totally clear up. We hope J-Biebs will be posting another set of snaps later this week showing us that his eyes are healthy and all healed up again!