Also available are pitchers that filter a small amount of drinking water and hold it chilling in the refrigerator. Water from the tap flows over charcoal, and contaminants adhere to the surface of the charcoal. They're generally good at removing some lead and chlorine and chlorine by-products left when municipal water systems treat water for germs. Carbon-based filters usually have removable cartridges that must be changed. If you keep using a filter after the cartridge is depleted, you're probably doing yourself more harm than good.
Some filters are better than others at removing bacteria from water.
Cryptosporidium is rare in water supplies, but when it appears it can cause healthy people to suffer gastrointestinal illness for a few days. It can be fatal for people whose immune systems are weakened. If you're concerned about bacteria in the water, carefully read the package of the filter you're thinking of purchasing to confirm that it is designed for microbe filtration. NSF International, a nonprofit organization, certifies which contaminants various filter brands remove. Another source of information is the Water Quality Association, but remember that this agency is an industry group representing water-filter makers.