BACTERIA 379 



blood corpuscles. This occurs every other day in tertian fever, 

 and on every third day in quartan fever. A double inoculation 

 may result in a more complicated succession of fever days. 



771. The Apes, Bats and Birds are also subject to Plas- 

 modium parasites. Texas cattle fever is caused by a elated 

 Sporozoan (Babesia), which is transmitted by the cattle tick. 



772. Bacteria as Parasites. — Most infectious diseases are 

 caused by bacteria. This has been definitely established for 

 many diseases, but the difficulties in the way of determining a 

 causal relationship between such minute organisms and the 

 diseases with which they are supposed to be associated are 

 often very great, and in a number of cases the organism has 

 not yet been identified, though the disease is almost certainly 

 known to be bacterial. The part of the body infested by the 

 parasites varies with the species; sometimes it is the mucous sur- 

 faces of the digestive tract or the respiratory passages, some- 

 times the tissues of certain organs, sometimes the blood vessels 

 and lymph spaces of certain organs or even of the entire body. 

 The mode of infection also varies with the disease. Some- 

 times the germs find their way to the host with the food, water 

 or air of respiration, but they may also enter the body through 

 the skin. The latter is not likely to occur except when the skin 

 is broken. 



773. The effect of the parasite on the host is sometimes 

 limited to a disorganization of the tissues of a limited region. 

 The result of this may not be serious, but if the destruction 

 of the tissues goes far in a vital organ the function of the organ 

 may be seriously impaired and result in the death of the host. 

 In other cases no anatomical change can be observed in the 

 tissues, and yet the function of some organs may be disturbed 

 and consequently the life of the host threatened. 



774. Bacteria vary like other classes of parasites with regard 

 to the range of hosts in which they may be found. But this 

 question has been studied more particularly from the point of 



