As infection from without is the source of the main diseases of mankind, the growth of knowledge of infection forms a supremely important part of the history of preventive medicine. This study naturally divides itself into the period before and since the discovery of the micro-oganisms demonstrated to be the efficient agents in producing some of the infectious diseases. It is remarkable, as pointed out by Professor Karl Sudhoff, that although the theory of natural causation originated with the Greeks, the Greek physicians were blind to the fact of contagion or infection. The idea of infection developed in connection chiefly with leprosy, a disease usually associated with terrible cutaneous lesions. The second great battle of preventive medicine was fought with but little success against Plague. Sydenham had described the remote causes of epidemics as wholly inscrutable. Sylvius ascribed fever to increased velocity of the circulation.