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.
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.
WebMD Medical Reference from Healthwise