Anyway, I found what I was looking for; I had read an excerpt of a book called How Doctors Think, where the author discusses the availability heuristic:

In the theory, "availability" is defined as the tendency to judge the likelihood of explanation for an event by the ease with which relevant examples come to mind. In a clinical situation a diagnosis may be made because the physician often sees similar cases in his practice — for example, the misclassification of aspirin toxicity as a viral pneumonia, or the improper recognition of an essential tremor as delirium tremens due to alcohol withdrawal in an indigent urban setting. Groopman argues that a clinician will misattribute a general symptom as specific to a certain disease based on the frequency he encounters that disease in his practice.

http://en.wikipedia.org/wiki/How_Doctors_Think