

CHAPTER II. 



ORGANISMS SAPROPHYTES PARASITES GENERAL REMARKS 

 ON DISEASE-PRODUCING MICROORGANISMS. 



THERE are two classes of objects in nature, one alive and animated, 

 the other one inanimate. To the latter belong rocks, minerals, 

 chemicals, etc. If we examine an object belonging to this class, for 

 instance a piece of iron, we find that any part is similar in structure 

 to the whole block, and has all its properties. If, on the other 

 hand, we examine a live object, in other words a living being, we 

 soon notice that there are several, in fact many, parts unlike each 

 other in properties, and, of course, individually unlike the whole, and 

 that these different parts or portions perform different functions. 

 Such different parts of a living being are called its organs, and the 

 general scientific term for live objects is organisms. Modern studies 

 have shown that all organisms are built up of small component parts 

 called cells. Most organisms are composed of a multitude of cells, 

 but there are many which consist of only a single cell. These are 

 called unicellular organisms. Such very minute beings can be seen 

 only with the aid of the microscope, hence they are known as micro- 

 organisms, and, more popularly, often as microbes. 



We divide organisms into plants and animals, and an enumeration 

 of the differences between a higher plant, for instance a tree and a 

 higher animal, and for instance a horse, is easy. However, when we 

 come to the unicellular organisms of the lowest type it is sometimes 

 difficult to decide whether we are dealing with a minute plant or a 

 minute animal. There are, in fact, microorganisms which are classi- 

 fied as vegetables by some investigators and as animals by others. 



Pathogenic Microorganisms. The following pages will deal par- 

 ticularly with the microorganisms, both animal and vegetable, which 

 may invade the body of man and domestic animals, and may there 

 multiply, in this manner becoming the cause of the so-called infec- 

 tious diseases. Such minute disease-producing organisms, or patho- 

 genic microorganisms, belong to the various phyla, tribes, classes, 

 orders, and families. The common feature which makes them inter- 

 esting and important to the student and practitioner of human and 

 veterinary medicine is the fact that they are the cause of much disease, 

 suffering, loss, and death. It is well to point out in the beginning 

 that while certain of these pathogenic microorganisms cause disease 

 in man, and not in the lower animals, as, for instance, the microbes 

 of typhoid fever, leprosy, syphilis, etc., and while others cause disease 



