66o MICROBIOLOGY OF DISEASES OF. MAN AND DOMESTIC ANIMALS 



disease is an infectious disease which is transmitted from one individual 

 to another by contact. The term refers to the method of transmission 

 rather than to the cause of the disease. It is possible that certain conta- 

 gious diseases may be transmitted by indirect contact or by the agency 

 of fomites but many authorities now hold the view that these factors 

 are non-essential and that most contagious diseases are transmitted by 

 direct contact. 



MICROORGANISMS OF DISEASE CONSIDERED AND CLASSIFIED 



PATHOGENIC BACTERIA. Bacteria which produce disease are known 

 as pathogenic bacteria. Of the many thousand species of bacteria only 

 a comparatively few species have anything to do with the diseased 

 processes in the plant or animal body. Those bacteria which are 

 capable of growing in the body of animal or plant may be designated as 

 parasitic bacteria. Some bacteria can grow only in the animal or plant 

 body and do not exist for any period of time outside of it. They are 

 known as obligate parasites. There are others which may produce 

 disease in the animal or plant body which can grow and reproduce 

 outside the body. They are known as facultative saprophytes. There 

 are still other bacteria which ordinarily live outside the animal and 

 plant body and which exist largely upon dead organic material, which 

 when taken into the body occasionally produces disease processes. 

 They are called facultative parasites. As an example of an obligate 

 parasite the Bact. lepra of leprosy may be cited, although in this 

 instance certain observers have claimed to have cultivated the bacillus 

 in pure culture. However, the results are not in any sense uniform. 

 Improved bacteriological technic has made possible the cultivation of a 

 large number of bacteria which heretofore were regarded as obligate 

 parasites. As examples of facultative saprophytes the B. typhosus of 

 typhoid fever and the Msp. comma of cholera may be mentioned. 

 As examples of facultative parasites B. tetani of tetanus and Bact. 

 wclchii of gaseous gangrene may be mentioned. 



PATHOGENIC PROTOZOA. There are several infectious diseases in 

 man and animals which are caused by pathogenic protozoa. Among 

 the common diseases due to protozoa there may be mentioned malaria, 

 syphilis, rabies (the nature of the organisms involved in syphilis and 

 rabies is not well understood however), amoebic dysentery, Texas fever, 

 infectious jaundice of dogs, and the various trypanosome infections 



