Diseases which are due to unicellular pathogenic microörganisms are called _infectious_ diseases, while if such diseases are transmitted under natural conditions from organism to organism they are spoken of as _contagious_ diseases. Most infectious diseases are contagious but not all. Tetanus is a good illustration of a non-contagious infectious disease. There are very few such diseases. When a unicellular microörganism gains entrance into the body and brings about any pathological changes there, the result is an _infection_. Undoubtedly many pathogenic organisms get into the body but never manifest their presence by causing disease conditions, hence do not cause an infection. It is the pathological conditions which result that constitute the infection, and not the mere _invasion_. The time that elapses between the entrance of the organism and the appearance of symptoms is called the _period of incubation_ and varies greatly in different diseases. The term _infestation_ is used to denote pathological conditions due to _multicellular_ parasites. Thus an animal is _infested_ (not infected) with tapeworms, roundworms, lice, mites, etc. Many of these conditions, probably all, are contagious, _i.e._, transmissable naturally from animal to animal. The word _contagious_ has been used in a variety of ways to mean _communicated by direct contact_, communicated by a living something (_contagium_) that might be carried to a distance and finally _communicable_ in any manner, transmissable. The agency of transmission may be very roundabout--as through a _special tick_ in Texas fever, a _mosquito_ in malaria, etc.,--or by direct personal contact, as generally in venereal diseases. After all, though exactness is necessary, it is better to learn all possible about the _means of transmission of diseases_, than quibble as to the terms to be used. An infectious disease may be _acute_ or _chronic_. An acute infection is one which runs for a relatively short time and is “self-limited,” so-called, _i.e._, the organisms cease to manifest their presence after a time. In some acute infections the time is very short--German measles usually runs five or six days. Typhoid fever may continue eight to ten weeks, sometimes longer, yet it is an acute infectious disease. It is not so much the time as the fact of _self-limitation_ that characterizes acute infections. In chronic infections there is little or no evidence of limitation of the progress of the disease which may continue for years. Tuberculosis is usually chronic. Leprosy in man is practically always so. Glanders in horses is most commonly chronic; in mules and in man it is more apt to be acute. Many infections begin acutely and later change to the chronic type. Syphilis in man is a good illustration.