In the English language the word infection may mean not only the infectious agent itself but also its entrance into the body and all the consequences of such entrance. Latent infection, generally speaking, means the residence in the body of a specific infectious agent without any manifest symptoms. The symptomless incubation period, which in certain diseases, notably measles and smallpox, is fairly definite in length, is a period of latency in infection. After complete recovery from an infectious disease the infectious agent may survive in the patient for a variable time, sometimes for years, without causing any obvious disturbances. Here the infection outlives the disease. This form of latency in infection occurs in typhoid, cholera, epidemic meningitis, diphtheria, scarlet fever and other diseases. In malaria, recurrent fever, undulant fever and septicemic infections occur symptomless intervals or periods of latency the nature of which presents many problems. There are also infections