Bacteria and viruses in water are generally the result of compounds released due to decayed vegetation and are typically associated with different forms of algae. They are most prevalent in supplies that use surface water as their supply. While rarely toxic, they are nonetheless unpleasant and can be offensive at a very low concentration.
Rising temperatures, spring runoff and increased animal and bird activity all contribute to the rise in bacteria counts. Some of these microscopic creatures find their way into various water supplies, and multiply under favorable conditions.