Covid-19 can cause a diverse range of symptoms, affecting multiple body systems. Different people are affected in different ways: these varied manifestations have made it tricky to isolate cases and understand the underlying pathology. Importantly, some people may have no symptoms at all (asymptomatic) but can still spread the virus. There have been wide variations in the estimate for the proportion of COVID-19 carriers who are infected- understanding this is really important to reduce transmission.