Viruses are tiny (5–30 nanometres in size) infective agents that can grow only in living cells. The virus that causes smallpox, declared eradicated from the world in 1979, is illustrated in Figure 11.
Most viruses are able to remain viable in water at low temperatures, provided there is some organic matter present. Once excreted, the number of viruses cannot increase since they only multiply within living susceptible cells.
The main threat to water quality comes from human enteric (intestinal) viruses that are produced by infected persons and excreted faecally. Depending on local circumstances, this may contaminate river water directly or, if treated, via sewage effluent that may be discharged into a river. If the river water is abstracted and treated for drinking purposes, the viruses may not be completely removed. (It is possible for a person to be susceptible to only one viral particle.)
The presence of any enteric virus can be taken as an indication of the possible presence of other harmful viruses. In temperate climates, enteric viruses occur at peak levels in sewage during late summer and early autumn. The exception is the hepatitis virus, which increases in the colder months.