Having a disease or a symptom of disease. The term can also be used to refer to the amount of disease within a population.

Morbidity refers to disease, injury, and disability.  For example, the predominant illness or morbidity associated with COVID-19 is respiratory disease. Diseases that co-exist in a person are called comorbidities and include diabetes or heart disease. Comorbidities make people more susceptible to SARS-CoV-2 infection and severe COVID-19 disease.

Morbidity is measured in various ways. Researchers can measure the number of people in a population who become ill (incidence) or who are ill at a given point in time (prevalence) which gives an understanding of the magnitude of the disease.