According to US scientists from the University of Colorado, dirty shower heads can deliver a face full of harmful bacteria that causes lung disease.
(Levels of Mycobacterium avium on shower heads were 100 times higher than those found in typical household water supplies. Spurting water can distribute bacteria-filled droplets which suspend themselves in the air and can easily be inhaled into the deepest parts of the lungs. I'm just going to clean the shower now.)