and unpleasant, but not serious. Treatment should be left to a physician, and 

 persons who are infected should use care to prevent the spread of the parasite. 

 The condition known as "athlete's foot" is also due to a fungal parasite. The 

 treatment and prevention of the disease, and of others not caused by bacteria, 

 are possible because of knowledge derived from studies started by the germ 

 theory. 



Virus Disease By saving from rabies a little boy who had been bitten 

 by a mad dog, Pasteur convinced the world of the soundness of his germ 

 theory of disease (see page 444) . This brilliant achievement aroused tremen- 

 dous public interest and led to the establishment of the Pasteur Institute in 

 Paris, and later of similar institutes for research into the problem of disease, 

 in all parts of the world. But to this day nobody has yet seen the germ of 

 rabies, or hydrophobia. 



We have seen that in rabies, smallpox, and several other diseases the specific 

 cause of the disorder is a filterable virus (see pages 444-445). A virus is more 

 like a protein than like an organism, although it multiplies like a parasite at 

 the expense of the host. Virus diseases destroy plants, as well as animals; 

 and they arouse in the host reactions similar to those produced by injurious 

 bacteria. Infection by virus is also similar to that by bacteria. For these 

 reasons virus diseases are treated very much like bacterial diseases. 



Animal Microbes Many protozoa are parasitic. Malaria, dysentery, 

 syphilis, African sleeping-sickness, tick-fever in cattle, and other diseases in 

 man and the lower animals are caused by different species of protozoa. 



Many species of flatworms and roundworms live as parasites in the bodies 

 of higher animals. They are important to us because they injure either human 

 beings or domestic animals. 



A very striking fact in the life history of some of these animals is that dis- 

 tinct stages in the life cycle are passed in different hosts (see illustration 

 opposite). The same fact has been observed in many parasitic plants, as the 

 wheat rust, one variety of which spends part of the cycle on the wheat and 

 part on the barberry plant (see page 594). 



This fact of multiple hosts led to a great deal of confusion when scientists 

 first attempted to make a complete study of these parasite species. In the 

 end it turned out to be of great help in our struggle to overcome them, since 

 the more links there are in a chain, the better are our chances of finding one 

 that we can break. 



Parasite Worms The name tapeworm is applied to several species of 

 flatworms of the genus Taenia (see illustration opposite). It has a comparatively 

 simple structure, consisting of hardly more than a series of flat sacs containing 

 excretory tubes and reproductive organs, with a holdfast, or anchoring organ, 

 at the end (or rather the beginning) of the series. Three or four species of 

 tapeworms inhabit the human intestine. 



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