Elusive word and its meaning thereof...please help

I have been studying personality disorders, and the word comorbidity often arises in the texts thereof. I cannot find this word in any dictionary that I have looked in. (I have used almost a dozen, so far!) What does comorbidity mean? The following paragraph is an excerpt from one article about Avoidant Personality Disorder. I hope this helps in regards the context and usage of the word.

I found it by searching medical dictionaries. Here’s a quickie definition:

Here’s a page with a more involved one: http://www.graylab.ac.uk/cgi-bin/omd?comorbidity