22 TEXTBOOK OF BOTANY 



of the most important bacterial plant diseases will be dis- 

 cussed in Chapter XXIV. 



33. Tuberculosis. This name is given to diseased con- 

 ditions which are caused in various parts of the body by 

 Bacterium tuberculosis (Fig. g, A}. The most frequent form 

 is tuberculosis of the lungs, often called consumption. This 

 is the most serious and widespread disease that attacks the 

 human race. Each year about 150,000 persons are killed by 

 tuberculosis in the United States, and about 1,500,000 in 

 the world. Although the fight now being waged against 

 tuberculosis is reducing the death rate from year to year, it 

 is still true that of the persons now living in the United States 

 more than 5,000,000 will die of tuberculosis unless more 

 successful methods than are now known are found for treat- 

 ing the disease. 



34. How Tuberculosis is Caused. The lung cells that 

 are attacked by the bacteria are destroyed. Then there is a 

 growth of new cells around the diseased region, forming a 

 tubercle, which gave the disease its name. By the de- 

 composition of the lung tissues, the bacteria produce, among 

 other substances, one which passes into solution in the blood 

 of the patient and there acts as a poison. This is one of a 

 class of poisons, or toxins, which many parasitic bacteria 

 produce. The toxin of the tubercle bacteria is responsible 

 for many of the serious features of the disease. Eventually, 

 if the disease is not in some way checked,' the tissues of the 

 lungs are largely destroyed. In many cases, however, in 

 otherwise healthy and vigorous persons, the disease does 

 not progress far. This is because the white corpuscles of 

 the blood attack and kill the bacteria. The patient then 

 recovers. 



The possibility of a cure of tuberculosis in its early stages 

 shows the importance of keeping in good general health as a 

 precaution against the disease ; and also the necessity of be- 

 ginning the treatment of tuberculosis at the earliest possible 



