PROBLEM 1 How Are Our Bodies Protected Against 



Micf oorganisms? 



Disease. Without troubling ourselves 

 about an exact definition of the word 

 disease, we can think of a disease as a 

 condition in which all or some part of 

 the body is not working properly. Ear- 

 lier you studied certain diseases, such as 

 diabetes, which occur when the ductless 

 glands do not work normally. These can 

 be called inetabolic diseases. In the dis- 

 cussion of diet you read of deficiency 

 diseases, such as scurvy and rickets, 

 which occur when the diet does not con- 

 tain enougrh of the necessarv vitamins. 

 Another type of disease, organic disease, 

 is caused by the actual change of some 

 organ in such a way as to prevent it from 

 working well. Such diseases are heart 

 disease or kidney disease. 



This unit, however, is concerned with 

 a different type and a very common 

 type of disease in man. It deals with the 

 diseases caused by other living organisms 

 which make their home in or on man's 

 body. 



Parasites. Organisms that make their 

 home in or on some other organism and 

 that obtain their food directly from it 

 are called parasites; their type of food- 

 getting is called parasitism. 



There are many parasites that may 

 affect us. Some are fairly large, such as 

 the various kinds of worms that live in- 

 side us, but most parasites are micro- 



scopic. The microscopic parasites may 

 be protozoa (animals) or they may be 

 plants such as moldlike forms and bac- 

 teria. Ringworm, for example, is caused 

 by a moldlike plant and so, also, is ath- 

 lete's foot. Malaria is caused by a proto- 

 zoan. But parasitic protozoa and mold- 

 like forms are comparatively rare. By far 

 the largest number of microscopic para- 

 sites are bacteria. 



Since the organism in or on which a 

 parasite lives is called the host, we can 

 say that man is the host of many types 

 of bacterial parasites. Those that cause 

 disease in their host are called pathogenic 

 (path-oh-jen'ik), a word that means 

 "producing suffering." Aiicroscopic or- 

 ganisms that cause disease are popularly 

 called ger?ns. 



Although there are many kinds of bac- 

 teria that live as parasites in or on man 

 and other living things, not all bacteria 

 are parasites. Far from it. The majority 

 live on dead plant or animal matter or on 

 food. Such bacteria cannot be called 

 parasites. You will read about these in a 

 later unit. 



Characteristics of bacteria. Bacteria 

 (singular bacterium) are single-celled 

 organisms. The cell is usually very tiny, 

 much smaller than most plant or animal 

 tissue cells. Bacteria have a firm cell wall 

 and for this and certain other reasons 



