INFECTION 145 



terms have now to a laTge extent been discarded. Ex- 

 cluding individual susceptibility, 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 contagion gains access by the respira- 

 tory tract ; and (3) the relative virulence of the contagion ; 

 in some instances the smallest amount of the contagion 

 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 will obviously 

 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 diphtheria 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 infection, e.g. ringworm. 



Parasites may therefore be divided into infective and 

 non-infective, though there is a series of connecting links 

 between these, and the two groups cannot be sharply 

 separated. The infective parasites are : (1) vegetable 

 micro-organisms, chiefly bacteria, a few yeasts and some 

 moulds ; (2) many protozoa ; and (3) a few metazoa, 

 generally worms. The non-infective parasites are the 

 animal parasites generally, particularly many worms. 



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