32 IMMUNOLOGY 



Metastasis. — During the period of existence of a focal infection 

 or the course of an infectious disease or, for that matter, following- 

 major or minor operations, there may occur new foci of infection 

 in parts of the body not primarily involved. This extension of 

 organisms to new regions, which is called metastasis, may occur 

 either by way of the blood stream or lymphatics. This explains the 

 development of pneumococcus meningitis, typhoid meningitis, or 

 tuberculous meningitis associated with the respective diseases. It 

 likewise explains bone involvement (osteomyelitis) and inflamma- 

 tion of venous walls (phlebitis) during typhoid fever. Patients 

 operated upon for localized streptococcus infections in the abdomen 

 not infrequently develop phlebitis of the femoral or popliteal veins 

 of the left leg. It is upon this basis that one commonly explains 

 many cases of inflammation of the gall bladder (cholecystitis), 

 kidney (nephritis), pancreas (pancreatitis), etc. 



Secondary Infections. — Not infrequently a patient suffering 

 from a specific infectious disease may develop a secondary infec- 

 tion due to an entirely different organism. The first disease 

 lowered the patient's resistance and created a favorable soil for 

 secondary invaders. One sees this occasionally in measles, which 

 is apparently caused by a virus. These patients may develop a 

 secondary streptococcus invasion of the lungs, which is called a 

 streptococcus pneumonia. Secondary pneumonias of this type 

 may be due to any of a number of organisms found upon the 

 respiratory mucous membrane of tlie patient. In like manner 

 secondary involvement of other tissues may occur and complicate 

 the primary disease. 



Infectious and Contagious Diseases. — Any disease which is 

 caused by an infectious agent is called an infectious disease. When 

 it is quite readily communicated from one individual to another, it 

 is classified as a contagious disease. Thus it is apparent that 

 while all contagious diseases are due to infectious agents, not all 

 infectious diseases are contagious. 



Toxemic Diseases. — In a few diseases such as botulism, tetanus, 

 gaseous gangrene, and diphtheria, infection either does not occur 

 or is very slight. They are in reality toxemic in nature whereas 

 in scarlet fever and in staphylococcus and pneumococcus infections 

 there are both toxemia and severe infections. 



Bacterial Toxins. — Roux and Yersin (1888) were the first to 

 show that some bacteria secrete toxins having specific actions in the 



