A virus is a submicroscopic particle that contains either RNA (ribonucleic acid)or DNA (deoxyribonucleic acid). Viruses are not capable of performing metabolic functions outside of a host cell upon which the virus depends for replication. Viruses are found in the environment in a wide range of sizes, chemical composition, shape, and host cell specificity. Viruses can cause disease or genetic damage to host cells and can infect many living things, including plants, animals, bacteria, and fungi.

Bacteriophages are viruses which use bacteria as hosts. Of particular interest in the aquatic environment are coliphages, viruses that infect Escherichia coli, a bacteria that commonly grows in the colons of mammals. E. coli is an important bacterial indicator of fecal pollution of water. However, coliphages tend to survive much longer in the environment than E. coli, and their detection in water in the absence of E. coli tends to be...