Mouse protection tests with 8 viruses isolated recently in Uganda were done on the sera of 297 indigenous residents of Uganda and Tanganyika. One hundred thirty-eight sera failed to neutralize any of the viruses; 91 neutralized one or other of the 8 but no more than 1; 68 sera neutralized more than 1 of the 8 agents.

Only 1% of the sera neutralized Mengo virus, and it is believed that this agent is probably not very important medically in the localities sampled. Neutralizing antibody against all the other viruses was present in sera from various localities and areas remote from the point where each was encountered. The approximate order of prevalence of infection in human beings with these agents in the areas covered is as follows: Bwamba fever, Ntaya, Zika, Uganda S, West Nile, Bunyamwera and Semliki Forest. The incidence of humoral immunity to the first four named agents among these 297 donors is sufficient to indicate that they attack man quite frequently.

Analysis of data pertaining to sera which neutralized more than one virus indicates that infection with Bwamba fever virus occurs without relation to infection with any of the others, but that infection with Ntaya, Zika, Uganda S or West Nile virus is, for one reason or another, likely to be associated with infection by one of more others of this group. It is possible that similar correlations may exist for Bunyamwera and Semliki Forest viruses, but the number of sera protective against these agents was too small to establish this with certainty.

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