(Medical slang terminology)
There is in medicine, as in many professions, an internal language or unofficial terminology used by its practitioners as a way of communicating the common memes of that profession efficiently.
The term "ticks and fleas" is used most commonly to refer to the condition of a person who has a multitude of medical problems. When speaking on the phone to a surgeon to discuss a patient you are referring, you may refer to a person with COPD, CHF, and clinical depression who you are sending for suspected superior mesenteric artery syndrome as someone who has "ticks and fleas", meaning they have a multitude of illnesses or are in poor shape overall.
There is a more specific meaning, however, relating to the differential diagnoses a physician may have - this being that a patient has more than one of the conditions you thought they might have. If you see a dog scratching, the obvious thought is that the dog has ticks or fleas, but on checking, you may find that it has both. This illustrates the more accurate definition of the term.
An example: a patient with chest pain may have a differential of cardiac disease, reflux disease, or chest wall pain. If you find out later after testing that the patient does indeed have cardiac disease, but also had one or both of the other diagnoses, you would say that the patient had "ticks and fleas" (emphasis on the "and").
This is such a colorful and descriptive term, and one which conveys its intended meaning so well, that I can't help but feel that other professions could pick it up for application as part of their unofficial terminology.
See also Gomer - Turf - Zebra