220 DISEASES FROM HARMFUL ORGANISMS 



and they are therefore classed as uncertain. There are 

 also a great many organisms which can produce disease, 

 but are so minute as not to be visible even under the 

 highest powers of the microscope. Moreover, on account 

 of their small size they pass through very fine filters, a 

 circumstance which causes them to be grouped under 

 the head of filterable viruses, a virus being a fluid which 

 is capable of producing infection, and a filterable virus 

 one which can produce infection after being filtered. This 

 obviously means that the organisms are so small that 

 they can pass through the filter. 



3. Organisms that are undoubtedly animals. — Under 

 this head come both protozoa and metazoa. In later para- 

 graphs specific illustrations of both classes will be given. 



Bacterial Disease. — Several common diseases are of 

 bacterial origin ; among them may be mentioned tubercu- 

 losis and typhoid. 



Tuberculosis is due to the growth within the body of 

 tubercle bacilli (see Fig. 55) which commonly make their 



Fig. 55. — Bacteria That Caii.so Tuberculosis. 



way through the respiratory passages into the lungs, 

 where they establish themselves and form little hard 

 masses known as tubercles. Tuberculosis is not confined 

 to the lungs but may also affect almost any other part of 

 the body. What is now known to be tuberculosis of the 

 lymphatic system was formerly called scrofula, or " King's 



