730 MODE OF SPREAD OF INFECTIVE DISEASES. 



mission is common to all infective agents, we have still 

 to ascertain their natural mode of spread in each in- 

 dividual case, and especially whether and to what extent 

 the infective agents are transmitted from the sick to the 

 healthy under the ordinary conditions met with in 

 practice. 



With refer- The natural mode of transmission evidently depends 

 natural ^ode on whether the infective agents can leave the body of 



ue?tion d is the tlie sic k in a state ca P a ^ e of producing infection. If 

 whether they are given off from some diseased surface of the 

 ^producing 6 body in sufficient quantity, in a living state and suf- 

 gtaooff-to ficien % resistant, then the disease may be transmitted 

 the sick. directly from the sick to the healthy, and spreads under 



certain circumstances by contagion. 



Contagious If, on the other hand, the infective agents which are 

 ous'infec^fve reproduced in the body of the sick do not leave it at all, 

 diseases. or on jy i n a g^g i nca p a ble of causing infection, the 

 infective disease in question is non-contagious. The 

 infective agents which cause these diseases must be 

 located somewhere in the surroundings of man, from 

 whence they can penetrate into healthy individuals, and 

 where they can also probably continue to multiply. 

 Hence all the non-contagious infective agents are 

 capable of a saprophytic existence, and belong to the 

 group of facultative parasites (see p. 629). The most 

 important representatives of this group are the infective 

 agents of malaria. 



In the group of the contagious infective agents we 

 are at once struck by a remarkable variety in the 

 degree of contagiousness. This undoubtedly depends 

 in part on the unequal resisting power of the healthy 

 body against the various pathogenic agents, an influence 

 which will be dwelt on presently ; in part also, however, 

 on the quantity and the resisting power of the infective 

 agents which are given off from the diseased body. 

 Contagion by Some of these are very vulnerable and quickly die after 

 ' they leave the body, and therefore can only pass from 

 the sick to the healthy by direct contact. Others 

 retain their vitality in the surroundings of the sick for 

 some time after they are given off from the body ; they 



