MICROORGANISMS AS A CAUSE OF DISEASE 187 



the specific cause by clinical examination or from the general 

 character of the lesions produced. Such infections as those 

 produced by the common pyogenic bacteria are non-specific. 



Primary, Secondary, and Mixed Infections. Any disease caused 

 by a single organism may be termed a primary infection. One 

 organism may break down the body defenses and make infection 

 with another and second species easy. The occurrence of pneu- 

 monia as a sequel to measles in man, for example, would be termed 

 a secondary infection. Two or more organisms associated in 

 bringing about changes are said to cause a mixed infection, as is 

 frequently the case in wound suppuration. 



Classification of Disease Types. Certain disease organisms do 

 not frequently enter the general circulation, but proliferate in a 

 more or less circumscribed area. In some cases such an organism 

 produces toxins, which are absorbed into the blood-stream and 

 cause injury to remote tissues, as in diphtheria and tetanus. 

 Such a disease is called a toxemia. When, on the other hand, no 

 appreciable amount of toxin is produced, it is known as a phlo- 

 gistic disease or infection, such as wound suppuration and gonor- 

 rhea. A general invasion of the blood-stream is called a bacter- 

 emia. The term septicemia has come to be used synonymously 

 with bacteremia, although more correctly applied to those types 

 produced by pyogenic organisms. Sapremia is produced by the 

 absorption of poisonous putrefactive products or the destruction 

 of necrotic tissues by saprophytic bacteria. Pyemia is a metastatic 

 pyogenic infection. Diseases characterized by skin eruptions of 

 certain types are called exanthemata (sing, exanthema}. For the 

 most part, the causes of these, such as small-pox, chicken-pox, 

 sheep-pox, scarlet fever, etc., have not been certainly determined. 



How Bacteria Produce Disease. A few bacteria produce the 

 specific toxins which have already been discussed. Intracellular 

 poisons, called endotoxins, are produced by other forms, and are 

 freed only by the dissolution of the cell. Possibly, disease may 

 sometimes be produced by mechanical means. The mechanism 

 in other cases is imperfectly understood. 



Groups of Pathogenic Microorganisms, Exclusive of Protozoa. 

 The disease-producing organisms can best be studied after a 

 classification of the related forms into groups. The characters 



