Communicable diseases continue as a major cause of death and sickness in the world, including the United States. This fact deserves renewed emphasis in these times of great interest in the achievement of "positive health," with much attention to the more currently publicized environmental problems of urban congestion.
Not only do these diseases result directly in the specific communicable illness, but they are often responsible for secondary diseases such as rheumatic fever, nephritis, pneumonia, and encephalitis. In addition, they often aggravate or complicate existing chronic disorders. Some may be implicated in or contribute to the "degenerative" diseases such as arteriosclerosis and stroke, and also cancer, and several may play a causative role in final or terminal episodes of chronic diseases.
A large part of the ravages of many infectious or communicable diseases can be prevented by immunization. And certainly, much is so prevented. However, it is clear that even in