CONTAGIUM 99 



Resistance. — The body resists bacterial invasion and injury 

 in various ways. Fever which follows many infections gives a 

 temperature which is unfavorable to the invading bacteria and 

 which otherwise opposes the invasion. The white blood cells, 

 aided by natural substances in the plasma, destroy bacteria. 

 Gland secretions, such as gastric juice, are destructive. Vari- 

 ous antagonistic substances are naturally present in the blood 

 and are hurriedly increased when bacterial invasion occurs. In 

 response to the toxin, the body produces antitoxin, as in tetanus 

 or human diphtheria. Substances (bacteriolysins) are produced 

 which dissolve and destroy the invading bacteria. In various 

 ways, then, the body tries to check or destroy the bacteria and 

 neutralize their poisons. These ^ r*r\rc atf-af- 

 factors together constitute im- • COGGACEA& 



munity, which is, of course, of 



all degrees of extent and dura- Mi^ BACTtRIACE'Ac' 

 tion. 



Nature disposes of disease t~, <§PIRILLAC£AE- TYPES 

 germs in a variety of ways, /~% p /'-« w -w 

 principally by oxidation, by ^ 36 _ Geneeal Geoups op Bac . 

 the devitalizing effect of sun- tekia. Diagrammatic. (M. B. B.) 

 light, by scattering them over 



wide areas and by enormously diluting them by wind and 

 water. 



Classification. — There are three general family types of bac- 

 teria : — 



Coccacem. — These are spherical. A great many of the most 

 common diseases are caused by germs which belong to this gen- 

 eral family ; for instance, erysipelas and various types of blood 

 poisoning, abscesses and sloughing. 



Bacteriacece — These are rod-shaped germs. Among the com- 

 mon diseases of live stock which are caused by germs belonging 

 to this general family are tuberculosis, glanders, tetanus (lock- 

 jaw) , and hemorrhagic septicemia. 



Spirillaeece.— r nie individuals are rod-shaped, curved, or spiral 

 and may be short or long. The specific cause of Asiatic fever 

 (human) belongs in this general group. Either type may show 

 the isolated individual or groups adherent in chains. 



Filterable wrw?.— We have also a little understood group of 

 ■filterable and invisible viruses, i.e. viruses which are so small 

 that they pass through laboratory filters and are invisible under 



