Contamination by bacteria and viruses can occur on several levels. In the setting of the laboratory, the growth media, tissues and other preparations used for experimentation can support the growth of unintended and unwanted microorganisms. Their presence can adversely influence the results of the experiments. Outside the laboratory, bacteria and viruses can contaminate drinking water supplies, foodstuffs, and products, causing illness. Infection is another form of contamination.

Equipment and growth media used in the laboratory must often be treated to render them free of microorganisms. Bacteria and viruses can be present in the air, as aerosolized droplets, and can be present on animate surfaces, such as the skin and the mucous membranes of the nasal passage, and on inanimate surfaces, such as the workbenches in the laboratory. Without precautions and the observance of what is known as sterile technique, these microbes can...