LATENCY IN INFECTIVE DISEASE, a condition in which, infection having occurred, the manifestations of bacterial disease are postponed. In one sense, latency is an excessive pro longation of incubation period, but the following case, reported by Abraham in a discussion on the subject at the Royal Medical and Chirurgical Society (now Royal Society of Medicine) in 1896 is remarkable. A man became leprous 4o years after his last ex posure to the disease. During this period of 4o years he had lived continuously in England and had held no communication, even by letter, with lepers or regions in which leprosy is endemic. It must be assumed that in this case the leprosy bacilli were introduced into the body of the man at least 4o years before he manifested the disease, and that during that period they remained living and latent. In the disease of silkworms known as "pebrine" the micro organisms which cause the disease can actually be seen in the eggs laid by an infected moth. But though the eggs are kept
under conditions not unfavourable to bacterial growth the micro organisms in the eggs do not begin to develop until the changes occur which lead to the formation of a caterpillar. Hence a strict latency, as distinguished from an undue prolongation of incuba tion period, in infective disease, must be allowed. It is quite con ceivable that the balance between the micro-organism and the tissues in which it is deposited, may, on occasions, be such that the micro-organism neither dies nor exerts its vital functions, but remains, so to speak, in a state of suspended animation and ca pable of development, if, at any time, the opposing force exerted by the tissues becomes lessened. Clinically "latent" disease almost always becomes manifest at a time when the patient has been subject to depressing conditions of some kind. (For the meaning of the term "latency" in biology see HEREDITY.)