2 logs per week. Hence storage of the final effluent in an oxidative lagoon for 1-2 months is an inexpensive and effective way of inactivating viruses.

Hygienic measures obviously have less effect on the incidence of respiratory infections, although washing of hands contaminated with respiratory secretions from a nose or handkerchief is important in minimizing the risk of transfer of many respiratory viruses. Attempts to achieve "air sanitation" by filtration and/or ultraviolet irradiation in public buildings have proved to have only a marginal effect, although these measures are an important feature of the biosafety cabinets that are widely used in virus laboratories. Respiratory viral infections are probably more common now than they have ever been, because of growing populations and the constant and extensive movement of people within cities, from rural to urban areas, and internationally. For human respiratory viruses, the population ol the world now constitutes a single ecosystem, although seasonal differences between the northern and southern hemispheres affect the incidence of particular diseases, like influenza, at any particular time.

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