According to a CNET article, your showerhead could be loaded with germs and pathogens linked to the development of pulmonary disease. The pathogen in question, Mycobacterium avium, was found in hundreds of showers studied for the testing of this theory. Presently, cigarette smoke is the leading cause of pulmonary disease, but scientist say that your showerhead and indoor air quality could be an inhibitor as well. They do not have any actual number correlations between the pathogen and disease cases, but to be on the safe side, you should avoid plastic showerheads in favor of metal. It is probably a good idea to clean and replace your showerhead periodically.