How can being in the hospital with pneumonia get any worse? When you end up getting a skin infection at the same time, for example. If that happened, you’d be dealing with an instance of comorbidity: the simultaneous occurrence of two or more illnesses.

Comorbidity may be easy to understand as a concept, but it can be medically complex. In the broadest sense, it just means that someone has more than one illness or disorder at the same time. These can be physical or mental. For example, depression is often comorbid with anxiety: a large percentage of people who have symptoms of one show symptoms of both. Epilepsy is often comorbid with both migraine headaches and mood disorders like anxiety and depression. Comorbidity can be difficult to treat, because illnesses can impact each other, or a treatment for one may cause unintended effects for the other. When dealing with comorbidity, doctors have to consider medication interactions, like whether a patient taking migraine medication can also safely take depression medication. Comorbidity can also increase the risk for other diseases. For example, people who have both diabetes and high blood pressure are at an increased risk for cardiovascular disease.

Often, comorbidity is discussed not just in individual people but in groups of people. Patient A has depression comorbid with insomnia is an individual statement, but depression is often comorbid with insomnia describes a general pattern of comorbidity in many patients. It’s important to remember that just because two conditions often occur together doesn’t mean that one is causing the other or that they are caused by the same thing.

Did you mean morbidity?