Inflammation has become a bit of a dirty word. We blame it for a number of diseases. We try to eat foods that fight it. We take medication to tamp down the pain it causes.
But inflammation, when it functions normally, is a natural and helpful response by the body to protect us. It’s the alarm sounded when we are infected with a virus, and what helps bones heal in the days and weeks after breaking an ankle.
It’s only when it sticks around for too long — or appears when there’s no threat — that inflammation can become harmful.
The “good” and “bad” kinds of inflammation share some characteristics, but a major difference lies in how long it lasts.
This type of acute inflammation happens with all sorts of injuries and threats: a cut, a burn, a respiratory virus, food poisoning.
But if some part of this process goes wrong, it can cause chronic inflammation that lasts for months or even years.
Chronic inflammation is associated with a wide range of conditions, including asthma, obesity, Covid, dementia, heart disease and cancer.
Sometimes, that can happen if the body forgets to send the signals that tamp down inflammation after the threat is gone. In other cases, the original threat doesn’t entirely disappear.
There are even times when the body responds to a threat that isn’t there.
Over time, chronic inflammation can result in irreversible damage to tissue. In the case of inflammatory bowel disease, scar tissue can form as the body tries to heal the colon. Or, white blood cells may clump together to try to wall off damage. Both issues can make it harder for the colon to do its job.
With asthma, another chronic inflammatory disease, the walls of the airways stiffen and thicken over time, reducing air flow.
Chronic inflammation isn’t necessarily static: It might flare up in response to a trigger, like when a person with rheumatoid arthritis is more physically active. This can make symptoms, such as joint stiffness or fatigue, worse for weeks.
And unlike acute inflammation, which often brings a swift and significant influx of cells and inflammatory proteins, chronic inflammation can happen at low levels over time. In atherosclerosis, for example, plaque slowly develops and hardens in the arteries. The body keeps trying to clear that blockage, resulting in background inflammation that accumulates to cause damage.
Scientists don’t fully understand what triggers the initial inflammation in many diseases. They have hypothesized that it could be bacteria, as in the case of inflammatory bowel disease; or something from the diet or environment, as might happen with pollen or air pollutants that cause the airway irritation seen in asthma. Or it could be something completely harmless that the immune system perceives as harmful, as happens with autoimmune conditions like rheumatoid arthritis, when the body mistakenly attacks its own joints.
Whatever the cause, it’s that persistent response that turns inflammation from one of your body’s best defenses into one of its most formidable enemies.
Knvul Sheikh contributed reporting.
Nina Agrawal is a Times health reporter.
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