becomes a bit too much and a bit overwhelming. zoe mcnulty is one of around 20,000 people in northern ireland thought to have some form of long covid. but what is this illness? scientists are still trying to establish a clinical definition. but for now, the uk guidelines define it as a condition that develops during or after the initial infection, that lasts for more than 12 weeks, and its symptoms can't be explained by an alternative diagnosis. so far, the illness has had more than 200 potential symptoms linked to it. professor sir terence stephenson is carrying out research into long covid.