8 - TheCel 



intergradation exists between nonliving and 

 living forms of matter. 



The first virus was discovered by Iwanow- 

 ski in 1892. Iwanowski found that juice 

 squeezed from a tobacco plant afflicted with 

 mosaic disease (Fig. 1-3), after passing through 



Fig. 1-3. A tobaco leaf infected with the mosaic 

 virus. Note the dark diseased patches, which give the 

 leaf a spotted (mosaic) appearance. (Courtesy of 

 L. O. Kunkel, The Rockefeller Institute for Medical 

 Research, New York.) 



an extremely fine porcelain filter, still could 

 give rise to the disease if brought into con- 

 tact with a healthy plant. This was surprising 

 since the clear filtered juice did not contain 

 any panicles large enough to be seen with 

 any existing microscope. Previously in the 

 nineteenth century, Pasteur, Koch, Reed, 

 and others had demonstrated that many dis- 

 eases in plants and animals are caused by 

 microscopic parasites — such as bacteria and 

 protozoans — which invade the tissues of 

 other plants or animals. But in the present 

 century it soon became apparent that other 

 diseases must involve infective bodies much 



smaller and simpler than any known micro- 

 organism. Now, in fact, a fairly large number 

 of virus diseases are recognized. These in- 

 clude smallpox, infantile paralysis, influenza, 

 the common cold, and measles, for man; 

 swine influenza, hog cholera, and bovine 

 hoof and mouth disease for other animals; 

 and the bacteriophages (Fig. 1-4) and mosaic 

 infections of plants. 



One unique feature of the viruses is the 

 extreme smallness of the individual particles, 

 each of which can be identified as being a 

 complete virus unit. If one takes a fluid con- 

 taining bacteria and forces this fluid through 

 a porcelain filter (ultrafilter), the filtrate 

 obtained is found to be sterile, that is, en- 

 tirely free of bacteria. Apparently the pores 

 of such a filter are so small that they prevent 

 the bacteria from passing through. If, how- 

 ever, one ultrafilters a fluid containing the 

 particles of a virus — such as the juice that 

 can be squeezed from a tobacco plant in- 

 fected with the tobacco mosaic disease, or 

 the fluid derived from the brain of a monkey 

 infected with infantile paralysis — the virus 

 appears in the filtrate, quite undiminished 

 in quantity. 



Growth and Reproduction. Another im- 

 portant characteristic of the viruses is that 

 each possesses, under the proper conditions, 

 an unlimited capacity lor growth and re- 

 production. Take, for example, the virus 

 that gives rise to infantile paralysis in man 

 and certain monkeys. This virus can be 

 transmitted from monkey to monkev in end- 

 less succession, without any sign of limit. The 

 smallest quantity of fluid from the brain of 

 a diseased animal, implanted into the brain 

 of a health) monkey, leads in due time to 

 paralytic symptoms in the inoculated animal. 

 During the incubation period, the original 

 minute quantity of virus increases to a tre- 

 mendous extent. The virus spreads through- 

 out all parts of the nervous svstem. Finally, 

 every small fraction of the brain of the 

 newly paralyzed animal contains as much of 

 the virus as was originally introduced into 

 the one localized site of injection. 



