PATHOGENIC MICROBES— INFECTION 121 



Infection. 



Infection may be defined thus : the attack on one living 

 being by another which penetrates it and Hves parasitically at 

 its expense. 



It is simply one case of the universal struggle and com- 

 petition among the species. 



The conflict between the invader and the invaded resolves 

 itself into a question of nourishment and digestion. "The 

 parasite attacks by secreting toxic or dissolving substances, 

 and defends itself by paralysing the digestive and expulsive 

 powers of its host. This latter exerts a noxious effect on its 

 aggressor by digesting it or eliminating it from its body, and it 

 too defends itself by means of secretions." ^ 



Infection exists among the amcebse. An amoeba invaded 

 by the parasites described by Metchnikoff under the name of 

 Microsphara finally succumbs. Certain infusoria are infected 

 by Acinetians, which pierce their cuticle and invade them. 

 The green euglena is subject to infection by lower fungi of 

 the chytridian group ; they lose their green chromatophores 

 and become literally anaemic. Infectious diseases are not the 

 peculiar privilege of man and the higher vertebrates. 



On the great problem of the origin of microbes, their mode 

 of transmission, and the way in which they penetrate the body, 

 bacteriology and hygienic science have accumulated many 

 facts. 



The microbes inhabit the air, water, the soil, animals, and 

 plants, and gain access to the patient either directly by simple 

 contact with another patient or thanks to more or less numerous 

 and various intermediate agents. Contact is sufficient for the 

 transmission of measles, small-pox, and scarlatina ; these are 

 the contagious diseases properly speaking. The air may carry 

 the germ from one individual to another. In the air the 

 B. tuberculosis may float, attached to particles of dried dust 

 or to moist droplets, projected into the air by the patient 



' Metchnikoff, Pathologie comparde de P Infamtnation^ 



