Break an ankle and it may start to look like a tree trunk. Unsightly, yes, but this external swelling (inflammation) is a vital part of healing. But what happens when inflammation is internal, ongoing, and undetected? Low-level inflammation is one of the greatest health threats facing Americans, warn experts, and you may not even know you have it.

Chronic inflammation occurs when the immune system becomes hyperactive in response to irritants—from smoking, stress, or a poor diet—and no longer switches off its efforts to heal the body. These factors stimulate the release of chemicals that activate the immune system’s inflammatory response in an effort to undo the stresses placed on the body, says Samer Koutoubi, MD, PhD, professor in the Department of Nutrition and Exercise Science at Bastyr University.

Find out if you have inflammation by taking the CRP test.

C-reactive protein (CRP) accompanies inflammation, making it a great gauge for measuring inflammation levels in the body (see “Take the CRP Test,” page TK). “When CRP remains high and low-grade inflammation becomes constant and untreated, it weakens the body’s immune system and paves the way for many chronic diseases, particularly heart disease,” Koutoubi says. In fact, an eight-year study involving nearly 28,000 women published in the New England Journal of Medicine found that more than half of the women who eventually developed heart disease had high CRP levels, even when their LDL (“bad”) cholesterol levels were in a healthy range.

Smart diet and lifestyle changes can reduce and may even prevent heart-damaging inflammation, says Koutoubi. The first thing to consider is losing any excess weight you may be carrying around. Studies have shown that people with excess weight around the middle often have elevated CRP levels. Next, follow these four simple inflammation-taming guidelines.