You twist your knee, it inflames with fluid, protecting the damaged area until it heals.

You get a cough and cold, from germs or pollen or pollution: Inflammation develops in the mucous membranes in your airways, and these become a sticky trap for whatever is causing the problem.

With an infection in the gastrointestinal tract, you might experience mucus in the stool as the body tries to clear itself of the bug.

These are three forms of acute inflammation, where the body’s immune system responds to an emergency.

Sometimes the body overdoes it, and an acute inflammatory response can be overblown and dangerous. This was the situation for many people with severe cases of COVID-19. See our report from 2022.

In these cases, you might feel that these responses are adding more to your discomfort, than making you feel better. Then things tend to settle down and life goes on.

The mystery of chronic inflammation

Chronic inflammation happens when this emergency response stays in play, causing the body to remain in a constant state of alert. Over time, this chronic response damages tissues and organs.

It plays a part in chronic conditions such as rheumatoid arthritis and diabetes. Likewise, it emerges when you’re seriously ill or disabled with conditions such as cancer and stroke.

While acute inflammation trends to have easily identified symptoms (the pain, the swelling) chronic inflammation symptoms aren’t so obvious.

These can include fatigue, body pain, depression or anxiety, weight gain or weight loss, and persistent infections.

These symptoms can range from mild to severe and last for several months or years.

Why does chronic inflammation develop, and persist? In the main, it’s a mystery.

Here’s what is known.

In some people, not all, chronic inflammation can develop when acute inflammation (injury, infection) goes untreated.

Or you can develop a full-blown auto-immune disorder, where the immune system goes haywire and mistakenly attacks healthy tissue.

Pollution or exposure to toxic chemicals can trigger chronic inflammation.

Some cases of chronic inflammation don’t have a clear underlying cause.

There are also suspected risk factors for chronic inflammation. These include smoking, long-term regular alcohol intake and chronic stress.

More about how it happens

Dr Robert H. Shmerling is medical editor of Understanding Inflammation from Harvard Health Publishing and an associate professor of medicine at Harvard Medical School. In an explainer, describes chronic inflammation as follows:

“When inflammation gets turned up too high and lingers for a long time, and the immune system continues to pump out white blood cells and chemical messengers that prolong the process … From the body’s perspective, it’s under consistent attack, so the immune system keeps fighting indefinitely.”

When this happens, “white blood cells may end up attacking nearby healthy tissues and organs”.

For example, if you are overweight and have more visceral fat cells – the deep type of fat that surrounds your organs – “the immune system may see those cells as a threat and attack them with white blood cells”.

The longer you are overweight, “the longer your body can remain in a state of inflammation”.

He said research has shown that chronic inflammation is associated with heart disease, diabetes, cancer, arthritis, and bowel diseases like Crohn’s disease and ulcerative colitis.

Clues from COVID-19

 As we reported in 2022, COVID-19 patients are more likely to develop cardiovascular diseases and diabetes soon after infection.

The risks are significant, particularly in the first three months after catching the virus, but they tend to drop away.

It’s now well recognised that COVID-19, on top of its own set of symptoms, “can cause disease throughout the body, likely by triggering pathways that cause inflammation”.

The researchers from King’s College London recommend that “doctors advise their patients who are recovering from COVID-19 to reduce their risk of diabetes through a healthy diet and exercise”.

Which is precisely the same advice doctors tend to give patients with or at risk of chronic inflammation.

For more about chronic inflammation, see here.