Population density and immune status, vectors and virulence of infection, nutritional status, sanitation, genetic susceptibility and medical management of cases, are important factors influencing the incidence and/or severity of virus infections. Thus, the prevalence and clinical importance of virus infections and the need for antiviral drugs differ from place to place and from time to time. National and World Health Statistics of notifications of disease give some index of the incidence of infections but not all virus infections are notifiable. Such statistics can be misleading also through failures to notify from sloth on the part of the physician or, in the absence of pathognomonic symptoms or signs, from errors in diagnosis. Any assessment of the need for new antiviral drugs should consider the availability, safety, effectiveness and cost of alternative measures, including prevention of spread of infection by control of vectors, immunization by use of viral vaccines, or treatment with existing antiviral drugs. Early start of treatment of acute virus infections with existing drugs gives the best results and, where the clinical diagnosis is uncertain, accurate rapid virus diagnosis is of paramount importance. Many virus infections are asymptomatic or of trivial importance and without sequelae. However, new or improved antiviral drugs are needed for the prevention and/or treatment of a number of significant conditions caused by viruses which are not at present adequately controlled. These include upper and lower respiratory tract infections, influenza, chronic hepatitis, gastroenteritis, infectious mononucleosis, measles, rabies, haemorrhagic fevers and warts. Furthermore, such drugs might prove of therapeutic value in the prevention or treatment of virus-associated tumours, such as hepatoma, nasopharyngeal carcinoma, Burkitt's lymphoma, Kaposi's sarcoma and possibly carcinoma of the cervix.
