40 Proceedings of Indiana Academy of Science 



settle in the organs and may remain there for some time and then finally 

 pass out through the urine. It usually is secondary to the intestinal 

 route but still very important. This is true of malta fever and may 

 be of tuberculosis, typhoid fever and a host of other diseases. This 

 concerns mostly the kidney. 



V. Scaling. Although we do not yet know the specific organism 

 for smallpox and chickenpox, still there is now absolutely no doubt of 

 these diseases being spread from the scabs. 



VI. Blood. The blood may harbor bacteria, protozoa (malaria, 

 trypanosomes, etc.), spirochaetes or even invisible unknown organisms 

 (foot and mouth disease). Such blood leaving by hemorrhages or 

 abscesses may spread plague, glanders, anthrax, etc. When a blood 

 sucking insect (fly, tick, flea, mosquito), absorbs infective blood it be- 

 comes capable of transferring the infection to other animals. Usually 

 there is a specific carrier for each disease and organism (one species 

 and one genus). 



Individual Carriers. 



The first recognition of an individual carrier was by Pasteur and 

 Sternberg, 1881, in pneumonia. Now it is known that all of us often 

 have some dangerous organism in our mouth. It causes us no harm but 

 later it may reach a susceptible person and start the disease. 



Later (in the early nineties) in the study of diphtheria, it was 

 found that convalescents could carry the organism in their mouths for 

 months. Moreover, typhoid fever patients may continue to excrete the 

 bacillus for months and years through the feces and the urine. The 

 same is true of cholera, but usually the dangerous period is much shorter 

 than that of typhoid fever; only a matter of a few weeks. 



Again, in spinal meningitis and infantile paralysis, many apparently 

 healthy persons have been shown to have infected nasal discharges. 



In the same class is Texas fever of cattle. The animal recovers 

 from the disease but remains a carrier for life. 



This principle is nicely illustrated in malta fever. The organism 

 was known for 15 years (1885) before its life cycle was worked out. 

 Investigation proved that nearly all the goats on the island of Malta 

 were infected and the oi-ganism was isolated from the goat's milk. Need- 

 less to say the disease is prevented by simply heating the milk. 



Many of the infectious diseases may be prevented by other means 

 than those referred to. Experience has taught us that often an indi- 

 vidual who has had one attack of a disease is immune from a second 

 attack. This fact was observed from the earliest times especially in 

 relation to the plague at Athens and smallpox in China. 



The very first example of practical application of this was noted 

 in China at least 1100-1200 B. C. Here children were exposed to small- 

 pox or actually inoculated with the serous exudate or dried lymph. It 

 was hoped that the disease which ensued would be mild in form. They 

 then usually survived and were rendered immune, that is refractory to 

 the disease. There was no change in this method for 3,000 years. This 

 variolation — to produce a mild form of the disease — was introduced into 

 Europe by Lady Montague about 1720. 



