Comorbidity (say "koh-mor-BID-uh-tee") means that a person has two or more diseases at the same time. These are usually long-term (chronic) diseases that need treatment for a lifetime.
Examples of comorbidities include having both high blood pressure and diabetes, or having high cholesterol, heart failure, and diabetes.
Comorbidities can change treatment options. One disease can make another disease worse, and the total effect of all the diseases may be more than each one on its own.