172 A MANUAL OF BACTERIOLOGY 



conveyed by inoculation, and it is therefore infective only. 

 But the distinction between infectious and contagious is 

 mainly one of degree, and these terms have now to a large 

 extent been discarded. Excluding individual suscep- 

 tibility, the relative infectivity of a disease probably 

 depends on three factors : ( 1 ) the contagion is freely given 

 off aerially and is not destroyed thereby ; (2) the con- 

 tagion gains access by the respiratory tract ; and (3) the 

 relative virulence of the virus ; in some instances the 

 smallest amount of the virus is sufficient to infect. If 

 the contagion can gain access only through a wound or the 

 digestive tract, the chances of infection may be largely 

 reduced. In certain instances infection is conveyed by 

 an intermediary, e.g. the mosquito in malaria, and in 

 such cases infectivity wih 1 obviously partly depend on the 

 presence and abundance of the intermediary. Infection 

 is manifestly a part of the whole subject of parasitism, 

 which includes the animal and vegetable parasites which 

 develop in the animal body. If, however, the subject of 

 parasitism is considered more closely, it will be seen that 

 there is a vast difference between, say, a condition caused 

 by the echinococcus or by the round worm, in which the 

 effects are largely mechanical and in which relatively 

 little poison is produced by the parasite, and the disease 

 diptheria caused by the diphtheria bacillus, in which the 

 diphtheria bacilli have little or no action mechanically, 

 but elaborate virulent chemical poisons which cause a 

 general intoxication. Some parasites also may produce a 

 general infection, e.g. anthrax, others only a local infec- 

 tion, e.g. ringworm. 



Parasites may therefore be divided into infective 

 and non-infective, though a series of links connects 

 them, and the two groups cannot be sharply separated. 

 The infective parasites are: (1) vegetable micro-organ- 

 isms, chiefly bacteria, a few yeasts and some moulds ; 



