Not much. Rare beef occurs when the cooking does not denature the red myoglobin in the beef. In this country beef is inspected and certified so disease is unlikely from poorly cooked beef. It does contain a goody amount of nutritional protein, and a few vitamins like vitamin b-12 less common in vegetable sources. Rarely beef can be contaminated during processing and outbreaks of illness have occurred.
Parasites. Parasites like liver fluke, and bacterial infections are dangers of rare meat.