NEGATIVE AND POSITIVE PHASES. 377 



In some instances the nature of the virus used 

 is unknown, as in smallpox and hydrophobia; in 

 all probability, however, it consists of micro-or- 

 ganisms rather than of toxins alone. In the case 

 of typhoid, cholera, plague and other diseases of 

 known etiology pure cultures, living or killed, are 

 inoculated. Protection does not follow immedi- 

 ately on the inoculation. We are sufficiently fa- 

 miliar with this fact in relation to smallpox, in 

 which several days are required for the formation 

 of a protective amount of the antibodies. There 

 is reason to believe that the interval between the 

 inoculation and the appearance of antibodies is 

 characterized by a decreased resistance on the part 

 of the individual, so that during this brief period 

 he is unusually susceptible to infection. 



That period immediately following the injec- JJfJ^Vl? and 

 tion of a toxin or microbe, in which the quantity Phases. 

 of antibodies undergoes a temporary decrease, 

 Wright speaks of as the negative phase of the im- 

 munization; whereas that period marked by the 

 new formation of antibodies is called the positive 

 phase. The negative phase lasts from a day or 

 two to several days, depending on the quantity and 

 nature of the virus injected (typhoid). A second 

 injection should not be given during the negative 

 phase, since it causes a further decrease in the 

 antibodies and prolongs the phase. Wright speaks 

 of this as a cumulative negative phase. A cumu- 

 lative positive phase, marked by the formation of 

 larger amounts of antibodies, may be induced by 

 the proper spacing of a number of injections. 



In certain instances the nature of the anti- Nature of 

 bodies is known. In typhoid, cholera, plague and J 

 dysentery, for example, they consist of bactericidal 



