En español | If you’re like most people, you think you’re taking reasonably good care of your eyes. You don goggles when operating power tools and usually wear sunglasses on the slopes or beach.

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But at least 2 million Americans are on a path to losing their vision and are doing little about it. Why? Fully half of the 4 million Americans with glaucoma — a leading cause of blindness — are unaware that their vision is at risk.

"With glaucoma, if you wait until there is a symptom related to loss of vision or reduced vision, then you're in the later stages," says Dr. Eve J. Higginbotham, senior vice president and executive dean for health sciences at Howard University in Washington, D.C.

Don't wait until there is a problem. Especially if you are at added risk (see below), collar this thief before it robs you of your sight. Here are the things you need to know.

Q: What exactly is glaucoma?
A: Glaucoma is a condition that damages the eye's optic nerve, a bundle of tissue carrying information about your visual world from your eyes to your brain. This damage is usually — but not always — caused by the buildup of naturally occurring fluid within the eye. When fluid cannot escape through the eye's clogged drainage system, the resulting pressure causes the optic nerve to deteriorate, resulting in vision loss and blindness.

Q: What's my risk?
A: Anyone can develop glaucoma, but some people are at higher risk, including people 60 and older; those with a family member, especially a parent or grandparent, who have or have had the disease; and Latinos and African Americans. Indeed, African Americans are five times more likely than whites to be diagnosed with the disease and four times more likely to lose their vision because of it, according to Lighthouse International. Also, having severe nearsightedness, hypertension or diabetes puts you at higher risk because these conditions can contribute to increased eye pressure that damages the optic nerve.

Q: What symptoms should I watch out for?
A: There are two main types of glaucoma, each with different symptoms. In open-angle glaucoma, the most common type, the eyes' drainage system becomes blocked slowly over time, so symptoms are not always pronounced. Gradual loss of peripheral vision is the primary symptom (click on the photo, above, to see what that's like). But often individuals who have the disease experience no symptoms at all. In closed-angle glaucoma, the eye's drainage system becomes blocked suddenly, causing acute, severe pain in one eye. Other symptoms: Nausea, decreased vision, halos around lights and a swollen eye. Closed-angle glaucoma is an emergency and can lead to complete vision loss within 24 to 48 hours, so it's important to see a doctor right away.

Next: How to protect your vision.