Conjunctivitis, or "pink eye", is an irritation of the eye -- specifically, of the conjunctiva (singular; "conjunctivae" is the plural), which is the clear membrane that covers the eyeball and the inside of the eyelids. The conjunctivae protect the eyeballs from dust, dirt, and chemical irritants, but they themselves can become inflamed or infected.
There are many ways your eyes can become irritated. Often, something floating in the air can bother your eyes: these include smoke and dust, which irritate the eyes directly, or airborne allergens such as plant pollens and animal dander. Chemical vapours in the air can also irritate the eyes, as you've noticed if you cry when you chop onions.
Your eyes can also be irritated by bacterial or viral infections. Many common viruses, including the flu and adenoviruses, irritate the eyes as well as make you feel lousy in other ways. Bacteria can also infect the eye; bacterial conjunctivitis is often more serious than viral, but we can treat bacterial conjunctivitis with antibiotic eyedrops. Often, though, bacterial conjunctivitis improves in 4-5 days even without antibiotics.
Of course, as with many other infections, we can't tell if conjunctivitis is bacterial or viral unless we culture the goop coming out of the eye. In a lot of cases, though, we simply treat without cultures: this way, even if the conjunctivitis is viral, we can prevent bacterial "superinfection", such as periorbital cellulitis. (This is one of the very few exceptions to the rule that I never give antibiotics unless I culture first to see what I'm treating.) As always, prevention is better than treatment; good handwashing is the best way to help keep bacteria and viruses away from your eyes -- and prevent colds as well, since you can catch a cold by touching your eyes after touching something that was coughed on by a person with a cold. (One reason conjunctivitis is so common in day-care centers is that small children aren't very good at keeping their hands clean.)
Allergic conjunctivitis classically shows up as itching, sometimes with a little mucus. It's often seen only at certain times of year, and is very frequent in patients with hay fever. Your doctor or ophthalmologist can suggest eye drops that can help with these symptoms.
Chemical eye irritation can be caused by fumes, or by direct contact of chemicals with the eyes. The worst case is chemical eye burns, and the worst of these are caused by alkalis such as sodium hydroxide (lye, Drano®). If your child's eyes are exposed to chemicals, you must see an ophthalmologist immediately. Before you head for the emergency room, though, put your child under a water faucet and flush the eyes out with lots of water -- most of the damage to the eyes happens in the first few minutes, so the faster you flush out your child's eyes the better the eyes will be.
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