Depending on the data you look at, between 10 and 40 percent of people who get Covid will still have symptoms months later. For some, those symptoms will be modest. A cough, some fatigue. For others, they’ll be life-altering: Debilitating brain fog. Exhaustion. Cardiovascular problems. Blood clotting. This is what we call long Covid. It’s one term for a vast range of experiences, symptoms, outcomes. It’s one term that may be hiding a vast range of maladies and causes. So what do we actually know about long Covid? What don’t we know? And why don’t we know more than we do? Dr. Lekshmi Santhosh is an assistant professor at UCSF Medical Center, and the founder and medical director of UCSF’s long Covid and post-ICU clinic. Her clinic opened in May 2020 and was one of the first to focus on treating long Covid patients specifically. We discuss the wildly broad range of symptoms that can qualify as long Covid; the confusing overlaps between Covid symptoms and other diseases; whether age, race, sex and pre-existing conditions affect a person’s chances of contracting long Covid; why it’s so difficult to answer a seemingly simple question like, “How many people have gotten long Covid?”; what to make of a recent study that seemingly undermines the biological existence of long Covid; how worried we should be about correlations between Covid and medical disasters like heart attacks, strokes and abnormal blood clotting; and more.