xiii.] Causal connection with infectious disease. 145 



contagious, but requires the existence of previous miasmatic 

 infection in the person who is to take it. It should be ob- 

 served also, that up to recent times the term infectious disease 

 was only used when the exciting cause of infection, the con- 

 tagium or miasma, was only imperfectly understood. If disease 

 was caused by known parasites, such as lice or entozoa, trans- 

 ferable from one person to another or only to be procured in 

 certain localities with special characters, it was not called in- 

 fectious but parasitic. 



It was natural that some ideas should be entertained respect- 

 ing the general qualities of the unknown and invisible contagia 

 and miasmata, and it was assumed, not without reason, that 

 they were special particles of matter which were efficacious for 

 infection in a state of the most minute subdivision and in the 

 most infinitesimal quantities. 



The qualities of living beings were for a long time ascribed 

 by some observers to these infectious particles or contagia, as 

 we may now usually call them, and first of all in the period of 

 history from which we have received the terms 'contagium 

 vivum ' or ' animatum/ then used in a somewhat obscure and 

 indefinite sense. The traditional expression contagium vivum 

 received a more precise meaning in 1840 from Henle, who in his 

 ' Pathologischen Untersuchungen ' showed clearly and distinctly 

 that the contagia till then invisible must be regarded as living 

 organisms, and gave his reasons for this view. His argument may 

 be briefly stated in the following manner. The contagia have 

 the power, possessed as far as we know by living creatures only, 

 of growing under favourable conditions, and of multiplying at 

 the expense of some other substance than their own and 

 therefore of assimilating that substance. The quantity, certainly 

 minute, of the contagium which communicates infection to 

 any one who pays a hasty visit to a person suffering from an 

 infectious disease, has the power of multiplying enormously in 

 the body of the infected person, for the latter is able to give the 

 infection to an unlimited number of healthy but susceptible 



