COVID-19, caused by SARS-CoV-2 infection, is mild to moderate in the majority of previously healthy individuals, but can cause life-threatening disease or persistent debilitating symptoms in some cases. The most important determinant of disease severity is age, with individuals over 65 years having the greatest risk of requiring intensive care, and men are more susceptible than women. In contrast to other respiratory viral infections, young children seem to be less severely affected. It is now clear that mild to severe acute infection is not the only outcome of COVID-19, and long-lasting symptoms are also possible. In contrast to severe acute COVID-19, such 'long COVID' is seemingly more likely in women than in men. Also, postinfectious hyperinflammatory disease has been described as an additional outcome after SARS-CoV-2 infection. Here I discuss our current understanding of the immunological determinants of COVID-19 disease presentation and severity and relate this to known immune-system differences between young and old people and between men and women, and other factors associated with different disease presentations and severity.