20 MANUAL OF BACTERIOLOGY. 



Leeuwenhoek, a citizen of Delft, in Holland (1632- 

 1723), appears to have been the first who actually saw bac- 

 teria. Yeast-cells he certainly observed, besides making 

 many other contributions of great value to biology. Leeu- 

 wenhoek produced admirable lenses of high magnifying 

 power, and described what he witnessed with singular accu- 

 racy and enthusiasm. 



Even before this time men had sought to explain the 

 phenomena of infectious diseases by supposing the body 

 to have been penetrated by minute parasites, for example 

 worms. The spread of such diseases through a community 

 from a single center could readily be accounted for by the 

 multiplication of a contagious element, itself alive (con- 

 taghnn vivum). With increasing knowledge of the abund- 

 ance of microscopic life these speculations took firmer hold. 

 But long before their truth was finally demonstrated great 

 advances \vere made in the prevention of infectious diseases. 

 Much honor is due the clinicians whose accurate observa- 

 tions and foresight accomplished important results at an 

 early day, working with what now seems a very meagre 

 knowledge of the facts. 



The production of immunity against small-pox by inocu- 

 lation was first practiced in oriental countries. The method 

 had long been in use in the East, when in 1718 it was 

 brought to the notice of Europeans by Lady Montagu, wife 

 of the English ambassador at Constantinople. The proced- 

 ure consisted simply of the introduction of the virus of 

 small-pox by puncture of the skin. An attack of small-pox 

 resulted, which was much milder and far less dangerous 

 than the natural disease. 



Lady Montagu stated in a letter : " Every year thousands 

 undergo the operation; and the French ambassador says 

 pleasantly that they take the small-pox here by way of 

 diversion, as they take the waters in other countries." The 



