340 



Constant 



century that Dr. Ross Harrison discov- 

 ered the technique of raising tissues out- 

 side of the human body. Now, tissues of 

 many kinds can be raised. Once tissue 

 culture had been perfected, experiments 

 with viruses made great strides. 



Is a virus living? Scientists do not all 

 agree on the answer to this question. The 

 virus causing a disease (mosaic disease) 

 in the tobacco plant was separated from 

 the plant by Dr. W. M. Stanley, one of 

 our Nobel prize winners. He proved that 

 the virus, when purified, formed protein 

 crystals, apparently nonliving, although 

 .it grew and reproduced in the plant. But 

 many scientists still think a virus is alive, 

 because under favorable conditions it 

 multiplies, as living things do, and keeps 

 its power of infecting organisms. Small- 

 pox virus has been grown on a tissue cul- 

 ture, a very tiny amount transferred to a 

 second culture, and so on, eleven different 

 times. At the end of the eleventh transfer 

 there was 50,000 times as much virus as 

 there was at the beginning of the experi- 

 ment. We cannot yet be certain whether 

 or not some of the viruses are living or 

 lifeless. If you are especially interested, 

 you will enjoy reading more about viruses 

 in some college bacteriology textbook. 



Little progress in conquering virus dis- 

 eases. More than 70 diseases attacking 

 man and other animals are known to be 

 caused by viruses. Among the common 

 ones besides smallpox are chickenpox, 

 poliomyelitis (infantile paralysis), epi- 

 demic influenza, mumps, measles, "par- 

 rot fever" (psittacosis), yellow fever, 

 and some very ordinary ailments, such 

 as the common cold and fever blisters. 



It is interesting to note that a virus dis- 

 ease, smallpox, was the first disease 



Care Is Needed for Health unit vi 



against which successful active immu- 

 nization was practiced. Jenner did not 

 know anything about either bacteria or 

 viruses. Pasteur showed how to immu- 

 nize against a second virus disease, ra- 

 bies. Since then relatively little progress 

 has been made in developing immuniza- 

 tion against diseases caused by viruses, 

 although there is now a vaccine against 

 two other virus diseases: influenza, a 

 contagious disease found all over the 

 world; and yellow fever, a serious dis- 

 ease of the tropics transmitted by mos- 

 quitoes. In many countries viruses are 

 being raised and experiments are being 

 carried on. Scientists believe they know 

 the reasons for many of their failures. 

 Success, therefore, may be just around 

 the corner. You can summarize all that 

 you have read about viruses by doing 

 Exercise 2. 



A discovery which seemed promising. 

 At the Pasteur Institute in Paris, a 

 Frenchman, D'Herelle (De-rell'), found 

 something small enough to pass through 

 a porcelain filter that preyed on and de- 

 stroyed larger bacteria. He called it bac- 

 teriophage (back-teefiy-o-faj), bacteria 

 devouring. Sometime before this an Eng- 

 lish scientist (Twort) had noticed that 

 in harmless colonies of bacteria there 

 were transparent, glassy spots where the 

 bacteria Mere disappearing from the agar. 

 Whatever it was that did this could be 

 transferred to other colonies. He pub- 

 lished his findings but did not attempt 

 an explanation. But D'Herelle continued 

 his studies and found bacteriophage 

 widely distributed. It is plentiful in sew- 

 aq;e. Therefore D'Herelle filtered the 

 sewage and collected the bacteriophage. 

 He hoped that it would kill the bacteria 



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