154 LOUIS PASTEUR 



There were other analogies. The disease popu- 

 larly known as the itch had been shown to be 

 caused by a minute mite which had the disagreeable 

 habit of burrowing into the skin and setting up a 

 peculiarly annoying type of irritation. This mal- 

 ady, which is highly contagious, was formerly much 

 more prevalent than is happily the case now. A 

 medical treatise written as late as 1833 stated that 

 the cause of the itch is entirely unknown and, 

 Hahnemann, the celebrated founder of homeopathy, 

 affirmed that three-fourths of human ills were 

 nothing more than the itch struck in ("gale 

 repercutee"). Here was a perfectly definite and 

 clearly demonstrated case of a contagious disease 

 caused by minute organisms which could be dug 

 out of the skin by a needle and observed to scram- 

 ble about in the field of the microscope. Another 

 "disease" caused by an animal parasite had been 

 made known through the discovery of the minute 

 worm, Trichina spiralis, which has the habit of 

 burrowing into muscular and other tissues and 

 lying coiled up in its so-called cysts. A disagree- 

 able disease of the scalp called favus, was shown 

 to be caused by a fungus growing in the skin. The 

 rapidly growing knowledge of the numerous para- 

 sites, large and small, that infest animals and 



