790 General and Applied Biology 



duce themselves; (4) in their resistance to chemical and physical agents, 

 they are intermediate between the resistant bacterial spores and the non- 

 spore-bearing baciUi; (5) the different viruses vary in size, some being 

 about 10 millimicrons and others as large as 200 millimicrons (a milli- 

 micron is one-thousandth of a micron, and a micron is one-millionth 

 part of a meter) ; (6) viruses cannot he grown in a strictly artificial 

 culture medium, but they must be grown parasitically in a cell for which 

 they are more or less specific; (7) they are invisible when using an ordi- 

 nary high-powered optical microscope employing ordinary visible light, 

 because the smallest particle visible under these conditions is about 0.2 

 micron in diameter, but they can be photographed with an electron 

 microscope; (8) they pass through filters of certain types and under cer- 

 tain conditions; (9) certain viruses produce animal diseases, such as 

 smallpox, measles, etc., and plant mosaic diseases, which are highly in- 

 fectious; (10) immunity to virus diseases in animals appears in general 

 to be rather permanent, as smallpox, chicken pox, mumps, etc. ; (11) 

 pathogenic viruses usually attack one set of tissues, the two most char- 

 acteristic tissues attacked being the skin (by dermotropic viruses) and the 

 nervous system (by neurotropic viruses); (12) viruses also tend to vary 

 (mutate) as a result of which some of them may change their disease- 

 producing capacity; in fact, new virus diseases in plants are appearing 

 continually; (13) according to one theory, viruses are nonliving chemical 

 substances, possibly autocatalytic enzymes, or "wild genes," because they 

 are protein, progagate themselves, etc. 



The following are some of the more common diseases caused by patho- 

 genic viruses: 



Smallpox (variola) 



Cowpox (vaccinia) 



Chicken pox (varicella) 



Mumps 



Foot and mouth disease (cattle and man) 



Common colds and influenza 



Yellow fever 



Dengue fever ("breakbone fever") 



Pappataci fever (three-day fever) 



Infantile paralysis (poliomyelitis) 



Rabies (hydrophobia) 



Epidemic encephalitis 

 Warts (various types; some infectious) 

 Trachoma ("granulated eyelids") 

 Hog cholera 



Parrot fever (psittacosis) 

 Dog distemper 

 Herpes zoster (shingles) 

 Herpes labialis ("cold sores") 

 Herpes febrilis ("fever blisters") 

 Mosaic disease of plants (tomato, po- 

 tato, tobacco) 



QIESTIONS AND TOPICS 



1. Describe the beginnings of agriculture in the distant past. What were the 

 first attempts at plant cultivation and animal breeding? 



2. Discuss the dependence of man upon the soil, both in the past and at the 

 present time. 



