We think of "control" of infectious diseases at several levels. In the United States, cholera, smallpox, yellow fever, and epidemic typhus fever no longer exist at all, although they are still present elsewhere in the world. Polio and typhoid fever, bubonic plague and diphtheria, all continue in this country, but at varying low levels. Compared with their depredations in the past, they are effectively under control. The term does not imply that efforts to keep them so can be relaxed or abandoned; on the contrary, it is fully recognized that any of them can come back as soon as carelessness creeps in. Nor is this merely an abstract principle, as recent outbreaks of cholera in southern Russia, the Middle East, and Africa, and of diphtheria in Texas, remind us.

Control, then, does not necessarily mean elimination of a disease, even though elimination may remain as the ultimate goal. The control of VD would not necessarily entail disappearance of the last instance of infection. It might call for reduction down to very low levels of prevalence with effective surveillance of the few remaining patients with gonorrhea and infectious syphilis, much as typhoid carriers have been kept under observation since epidemics were controlled by purification of drinking water. Today when typhoid fever shows itself, it is always as a small outbreak caused by contamination of food by a carrier. We might set our sights provisionally at a similar level of VD control.

Effective control measures for infectious diseases fall generally into two groups: sanitation and immunization. Sanitation meaning, broadly, destruction of the agent of the disease in the environment is the means used against cholera, yellow fever, typhus fever, typhoid fever, and bubonic plague, among the examples already mentioned. Immunization has been responsible for control of the others, and for an assist in some of the members of the first group. Sanitation is the more positive of the two measures where it can be applied. It eliminates the microbe or virus before it can reach its human host. Immunization, apart from its accessory role in cholera, typhus and typhoid fever, and plague all microbic diseases is most dependable in virus diseases smallpox, yellow fever, and polio, or in the few diseases due principally to bacterial toxins, of which diphtheria is the single example in our group. Yellow fever actually belongs equally in the sanitation and immunization categories. In more developed parts of the world it is spread by easily controlled mosquitoes, and sanitation in this instance, elimination of the mosquito is therefore the mainstay of control. In tropical forests and areas adjacent to them, other mosquitoes, much more difficult to deal with, carry the disease; and for control in such places a first rate vaccine is available.