COVID-19 has existed for less than six months, and it is easy to forget how little we know about it. The standard view is that a minority of infected people, who are typically elderly or have preexisting health problems, end up in critical care, requiring oxygen or a ventilator.

About 80 percent of infections, according to the World Health Organization, “are mild or asymptomatic,” and patients recover after two weeks, on average. Yet support groups on Slack and Facebook host thousands of people like LeClerc, who say they have been wrestling with serious COVID-19 symptoms for at least a month, if not two or three. Some call themselves “long-termers” or “long-haulers.”

Most have never been admitted to an ICU or gone on a ventilator, so their cases technically count as “mild.” But their lives have nonetheless been flattened by relentless and rolling waves of symptoms that make it hard to concentrate, exercise, or perform simple physical tasks.

Most are young. Most were previously fit and healthy.

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