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 THE BACILLUS OF~ TUBERCULOSIS 301 



contain tuberculous toxins which cripple the activity of the sperma- 

 tozoa. 



Length of Time Tubercle Bacilli Remain Virulent in Sputum. According 

 to experimental investigations, the virulence of dried tuberculous sputum 

 is not suddenly but gradually lost, a certain proportion of it retaining 

 its specific infective power under ordinary conditions, as in a dwelling- 

 room, for at least two or three months, and occasionally for a year or 

 more. 



Attenuation. Tul>ercle bacilli when subjected to deleterious influences 

 or to growth on culture media gradually lose their virulence. A culture 

 which Trudeau has grown on suitable media for ten years is no longer 

 capable of causing tuberculosis in healthy guinea-pigs, although origi- 

 nally virulent. Cultures grown at temperatures of 42 C. become atten- 

 uated more quickly. 



Mixed Infection. In regions where tuberculous processes are on the 

 surface, such as lung and skin infections, and also when the infection 

 itself is multiple, as in disease of the glands of the neck from tonsillar 

 absorption, the tubercle bacilli are usually associated with one or more 

 other varieties of organisms. Those of most importance are th'e strepto- 

 coccus, pneumococcus, and influenza bacillus. Besides these many 

 other varieties are met with occasionally in individual cases. What the 

 influence of this secondary or mixed infection is, under all circumstances, 

 is not exactly known; but generally the effect is an unfavorable one, 

 and not infrequently after a time the disease takes oh a septicaemic 

 character. For the technique employed in examining sputa for mixed 

 infection see page 312. 



Immunization. As in other infectious diseases, many attempts have 

 been made to produce an artificial immunity against tuberculosis, but 

 so far the results have been only fairly satisfactory. The great majority 

 of mankind has in a varying degree a natural immunity against tuber- 

 culosis. In many individuals this immunity is only relative, and is 

 maintained only so long as the health is kept at a high standard or 

 the exposure to infection not too intense or prolonged. An unfavorable 

 environment, the occurrence of some other infectious disease, overwork, 

 dissipation, or, in fact, anything which tends to depreciate the nutrition 

 of the body, is apt to render the individual previously immune susceptible 

 to the tubercle bacillus. 



Acquired immunity against many bacterial diseases is acquired 

 within a few days or weeks after the development of infection. This 

 immunity may be complete or slight and vary greatly in its dura- 

 tion. There is little in the clinical history of tuberculosis which 

 shows that acquired immunity occurs in this disease, for relapse is the 

 rule, and one attack does not seem to afford any protection against a 

 later one. For this reason the production of an artificial immunity 

 against tuberculosis has always been looked upon as a result possibly 

 never to be achieved. The careful study of tuberculosis seems, how- 

 ever, to indicate an attempt on the part of nature at the production of 

 acquired immunity in this disease. It is thought that from 30 to 60 



