The majority of people who suffer from Covid-19 fully recover. Millions of others find complete healing to be frustratingly elusive. A subset become gravely ill because of their infections. There is no single explanation, diagnosis or treatment that can be applied to the growing group of people affected. They are known as long-haulers and reflect the burden of the Pandemic on societies and economies.

There is no universally accepted definition yet. According to the World Health Organization, people who have had Covid 19 can have symptoms for at least two months and can't be explained. Even if there are no noticeable symptoms, anilments may persist from the initial illness or appear after the acute phase of the disease. They can also change. Alternative definitions have been proposed by other groups. The U.K.'s National Health Service suggests referring to symptoms that last more than four weeks as Covid syndrome and post-covid syndrome. Another definition may be needed for children.