A common hazard of household water is contamination by potentially harmful bacteria and other microorganisms. Short term gastrointestinal disorders and illnesses such as gastroenteritis, giardiasis, typhoid, dysentery, cholera, and hepatitis have been linked to water contaminated by microorganisms.

It is the responsibility of water companies to ensure that all water supplied is disinfected and does not contain microorganisms which are harmful to humans. Yet in 2012 several water companies were fined thousands of pounds for failure to disinfect water and also for supplying water that was unfit for human consumption. It may be the responsibly of these companies to do their job in disinfecting water supplies, but when they fail, it is the consumer who pays the price.

Recently the BBC published an article regarding the dangers of bacteria which lives on shower heads, they reported that:

"Tests revealed nearly a third of devices harbour significant levels of a bug that causes lung disease."
"Lead researcher Professor Norman Pace, said: "If you are getting a face full of water when you first turn your shower on, that means you are probably getting a particularly high load of Mycobacterium avium, which may not be too healthy."