Bacteria have been shown to live in "premises plumbing systems". The latter refers to the "pipes and fixtures within a building that transport water to taps after it is delivered by the utility". Community water systems have been known for centuries to spread waterborne diseases like typhoid and cholera. However, "opportunistic premises plumbing pathogens" have been recognized only more recently: Legionella pneumophila, discovered in 1976, Mycobacterium avium, and Pseudomonas aeruginosa are the most commonly tracked bacteria, which people with depressed immunity can inhale or ingest and may become infected with. Some of the locations where these opportunistic pathogens can grow include faucets, shower heads, water heaters and along pipe walls. Reasons that favor their growth are "high surface-to-volume ratio, intermittent stagnation, low disinfectant residual, and warming cycles". A high surface-to-volume ratio, i.e. a relatively large surface area allows the bacteria to form a biofilm, which protects them from disinfection.
The best way to find a good plumber is to ask friends and neighbors, remodeling contractors, and real estate agents. Nothing holds up a home sale faster than plumbing problems, so realtors often have a list of reliable, fast-response plumbers. Once you find a plumber you're satisfied with, keep his numbers (office, home, and cell phones, and beeper) handy for emergencies. This Old House: Dripping faucets, running toilets
Most states and localities require plumbers to be licensed. Although licensing requirements vary, most states and localities require workers to have 2 to 5 years of experience and to pass an exam that shows their knowledge of the trade and of local plumbing codes before they are permitted to work independently. In addition, most employers require plumbers to have a driver’s license.
Another way to avoid a service call from your plumber is to make sure the outside faucets are turned off in the winter and make sure you disconnect the outside hoses. You need to shut the water off from the inside. Then, open the valve on the outside to let the water that’s in there drain out—you switch both of them to the opposite direction so one is always closed and one is always open. We have to fix tons of these in the spring mostly because people leave their outside hoses connected and they freeze up. The repair could cost $100-$200 or more. Another tip would be if you’re going away for any length of time, like on vacation, turn off your water. If on any of those days the temperature drops below freezing, have someone check in on your house. I’ve been called to homes where the family returned from vacation, and there was water flooding out from the front door.