Biology of Man 517 



those due to the presence of living organisms or their products, and (2) 

 noninfectious (organic), or those due to a variety of causes other than 

 living organisms. Infectious diseases may be classed as ( 1 ) communica- 

 ble, or those transmitted naturally from one person to another and (2) 

 none om,m,unic able , or those not contracted from another infected in- 

 dividual. Hence, pneumonia, tuberculosis, etc., are infectious, com- 

 municable diseases, while tetanus (lockjaw) is an infectious, noncom- 

 municable disease. Sometimes the less desirable term, contagious, is 

 applied to infectious diseases which are transmitted by direct contact 

 ("catching"). Satisfactory progress has been made in this country 

 against the infectious diseases, as is shown by the mortality (death) and 

 morbidity (sick) rates. The noninfectious diseases include a wide variety 

 of abnormal conditions of body and mind caused by an even greater 

 variety of causes. Some of them such as pellagra, scurvy, etc. (discussed 

 elsewhere) are grouped as vitamin deficiency diseases; others are due to 

 chemical poisons; still others (sunstroke, concussions, frostbite, lacera- 

 tions, etc.) are due to physical agents; and still others, such as Bright's 

 disease (kidneys), cerebral hemorrhage, various types of heart diseases, 

 psychosis, and imbecility, are due to derangement of tissues. Upon the 

 basis of speed, diseases may be classed as (1) acute (sudden onset and a 

 short period of rather severe illness with subsequent recovery or death) 

 and (2) chronic (Gr. chronos, time) (gradual onset of symptoms and 

 prolonged illness) . 



The various types of living organisms which may cause infectious dis- 

 eases in man are bacteria, yeasts, molds, pathogenic protozoa, parasitic 

 worms, ticks, mites, etc. Before an infection can occur, certain condi- 

 tions must be fulfilled : ( 1 ) the living organisms must enter the body 

 in sufficient numbers, (2) they must enter the body through the right 

 channels, (3) they must maintain themselves in sufficient numbers to 

 cause the disease, (4) the body being entered must be susceptible to the 

 actions of the living organisms, and. (5) the invading organisms must 

 be sufficiently virulent or potent to produce the disease. 



Numerous discussions and examples of infectious diseases caused by 

 bacteria, yeasts, molds, pathogenic protozoa, parasitic worms, viruses, 

 etc., are given in various parts of this book, to which the reader should 

 refer. 



Since earliest times man has been afflicted by diseases and has theorized 

 as to their causes. The follov\'ing, by no means a complete list, will suf- 

 fice: (1) Demonic theory — the earliest primitive peoples believed that 

 disease was due to evil spirits or demons. Consequently, they tried to 



