Scant attention was paid to graded degrees of severity of infectious diseases in the past. Study was impeded by undeveloped knowledge, lack of exact methods of diagnosis, rigid clinical criteria of the "typical" case, and, during epidemics, by the pressure of the emergency. Inapparent infections were undetectable, and victims of mild attacks usually were ignored or regarded as having some other ailment. After 1868, for example, according to Wunderlich's dictum, typhoid was not typhoid unless fever reached 39.5 C (103.1 F) between the fourth and sixth days. During an epidemic of cholera as late as 1944, victims not dehydrated or in a shock-like state were not regarded as having cholera. Yet brief typhoid was known to Louis, and Koch had described mild cholera in 1892. I have gathered papers from which perti¬ nent information was drawn to form the basis of this essay. Statistics for leprosy, schistosomiasis, or ecchinococcosis requiring a life-time of observation were not encoun¬ tered. Those for syphilis are meager, but for tuberculosis they are better. Almost all the reports cited were made between 1940 and 1960, many of them stimulated by mili¬ tary exigency during World War II and