WITH THE WIDESPREAD USE of new techniques, a new era has emerged in the field of virus inves-tigation, and nearly one hundred antigenically distinct viruses have been isolated and identified since 1947. These" new" viruses include the poliomyelitis, Coxsackie and ECHO viruses, the croup associated (CA) virus of infants, myxoviruses such as the Sendai virus, haemadsorption (HA), adenoviruses and other respiratory viruses, and the virus of cytomegalic inclusion disease. These virusesare widely disseminated, there is a high incidence of carriers, and it is difficult to prove that the viruses isolated are not fortuitously present in the patient. Whereas the earlier phase of virus research might be described as coming under the heading of" Illness in Search of Viruses", the present position has aptly been described as" Viruses in Search of Disease". 4

It may be claimed that as viruses multiply only in the cells of the body they do in fact cause an infection even when no clinical illness is mani-fested. This problem is illustrated best by the enteric virus group, which includes as its members the poliomyelitis, Coxsackie and ECHO agents which are found in the intestinal tract of patients as well as apparently healthy persons. The problem also concerns the adenoviruses, isolated from tonsils and adenoids, and the herpessimplex virus. In the case of the enteric viruses, an isolation of the virus from the stool assumes greater significance if similar isolations can be made from the blood or sites remote from the gastro-intestinal tract such as the cerebrospinal fluid, brain or other tissues. Since there are many antigenic types of each of the individual members of the enteric group, the presence of antibodies to one or more of the serotypes, as a result of past infection, is common. It is necessary therefore to demonstrate a signi-ficant rise of serum antibodies to the infecting strain before concluding that the presence of anti-body indicates current infection. The relationship of a virus infection toa specified clinical illness may be difficult toassess even when a virus isolation has been obtained, and an im-munological response demonstrated. This problem presents itself when two illnesses occur at a rela-tively short interval, caused by two distinct viruses. The viruses of the enteric group persist in the stool for overa month after infection; it is easily seen that the occurrence of an additional infection by a closely related agent within that period of time