One of the surprising aspects of the pandemic is that symptoms can linger months after infection. This syndrome has been called "long COVID," and it's had a profound impact on many people's lives.

Scientists have begun to find abnormalities in the immune systems of some long-COVID patients that might help explain the syndrome, at least in some people. But there is still much more to learn.

The symptoms, such as headache, fatigue and cognitive dysfunction, persist or recur months after diagnosis, far more often than they do for the flu, researchers say.