Bacteria are small single-celled organisms. Bacteria are found almost everywhere on Earth and are vital to the planet's ecosystems. Some species can live under extreme conditions of temperature and pressure. The human body is full of bacteria, and in fact is estimated to contain more bacterial cells than human cells. Most bacteria in the body are harmless, and some are even helpful. A relatively small number of species cause disease.

Bacteria can be found indoors when “Black Water”, water from sewage, has entered the home from a pipe break, toilet overflow, storm water and other ways. Bacteria from black water should be considered hazardous and treated as such. It is important to have a professional clean and treat any areas, building materials, furnishings and personal items contaminated by black water.

Bacteria can also grow indoors from stagnant water or “Gray Water”. This type of water will be treated the same way “Black Water” is treated.