Often the air and water we encounter every day contain a variety of micro-organisms. We're lucky that most of these things are harmless and even the ones that aren't can be washed down the drain before they can make any trouble.
This is not always the case, however. It has been reported in a scientific journal that showers can pose a threat to a person's health. Actually, it's not the shower but the biofilm that collects in showerheads.
When this biofilm was studied for its genetic material, some samples had high concentrations of a microbe called Mycobacterium avium, which can cause respiratory illnesses in some people. This bug is sometimes found in tap water, but there it is not hazardous. However, if it becomes an aerosol, like when bug-infested water is forced through a showerhead at high pressure, it can become dangerous because its tiny particles can be inhaled. Once they're in the lungs, they can start an infection.
This is not a problem for healthy people. However, people with weak immune systems or those at high risk of lung diseases may want to take protective measures. Unfortunately, cleaning the showerhead with bleach doesn't get rid of the danger because the microbes just come back with a fresh flow of water. To help lessen the chance of an infection, you can let the water flow out of the showerhead for a bit and also replace plastic showerheads with metal ones.
adapted from an article in The Economist, September 19, 2009
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