Well water is ground water. That means it's been naturally cleansed, to some degree, as it moves through the earth. For this reason, ground water is usually free of organisms like Giardia and Cryptosporidium, though it could harbor viruses. Because of their microscopic size, viruses can move much further through the ground than most organisms. In other cases, wells can be contaminated by surface water: for example, from barn runoff, or when the well is too close to a stream or pond. If the well tests positive for coliform bacteria, it should be disinfected. Then, test it again. If bacteria is still present, the contamination is ongoing, and you'll need to find the source. Keep in mind, though, that coliform is not a definitive test. A well that has coliform bacteria may not necessarily be unsafe. On the other hand, a well could have no sign of coliform, and still contain dangerous viruses. Some experts recommend testing for E. coli bacteria, as they're a more specific sign of contamination. For more information on microorganisms in private wells, contact the county's agricultural department, or a water quality expert.