In epidemics there are always some cases that do not run true to form and consequently are difficult to diagnose. Yellow fever epidemics appear to be no exception to this rule. In the recent outbreak of this disease in San Salvador, Central America, there were many such cases in which several of the so-called classical symptoms were missing. Four of these, with their corresponding history charts, are presented in this paper.
Case I. L. H. D., a non-immune white male about forty years of age, had spent considerable time in the tropics and had taken part in two yellow fever campaigns in South America, but in neither of them had he come into close contact with infected patients or mosquitoes, and he had remained well throughout except for some gastro-intestinal disturbances and an attack of dengue in Barranquilla, Colombia. In Salvador, in the course of supervising some of the antimosquito work, he entered houses in the most heavily infected zone; also, he was lodged in a hotel immediately adjoining this zone.