VIII. VIRUSES AND INSECTS 185 



in plants recovered from virus diseases is an invariable and essential 

 factor for the immunity. Thus, freedom from a second attack of an 

 acute disease, or protection from the effects of virulent strains, persists 

 only as long as tissues are infected. As for insects, after they become 

 infectious on feeding on diseased plants, some remain capable for life 

 of excreting the virus. Namely, the insects are "immune" to the 

 virus for the remainder of their lives. 



Likev^^ise in the case- of animals, it is known that, although they 

 may apparently recover from infection vv^ith some viruses, the animals 

 •can continue to harbor the agent in their tissue long after recovery. 

 Accordingly, some workers have postulated that active immunity in 

 virus diseases is dependent on "infection-immunity", meaning that 

 resistance to reinfection is due to the persistence of living virus in 

 the body. The detection of viruses from recovered animals, however, 

 is not always possible unlike with plants. It should be considered, 

 therefore, that in the majority of animals, the structure of viruses in 

 the host cells may be altered after the recovery from the diseases so 

 as to be rendered unable to act as the virus itself but retaining the 

 ability to combine preferably with the virus without being suffered 

 from it. 



This concept is strongly supported by the fact ascertained with 

 phage ; bacteria may become immune to phage after the infection 

 ■with the phage but usually no phage is detected in the bacteria. 

 However, the phage will be released when the bacteria are brought 

 under peculiar 'conditions (7). It must be emphasized that the acquired 

 character to produce the virus is transmitted to progeny. In like 

 manner, human beings also may be able to transmit the virus-produc- 

 ing character acquired through the infection to the progeny. 



It may appear strange that bacteria that have become immune to 

 the virus will be lysed when the active virus is produced. As is 

 already suggested, this may be attributed to an unfolding of the 

 protoplasm protein structure which is necessary for the acquisition of 

 active virus structure. The virus structure in the bacteria may be 

 inactive because of the folding of the active group, and the unfolding 

 of the structure may result in the striking adsorption of water to the 

 surface of the elementary bodies leading to the dissociation of the 

 cell into the elementary bodies. This may be the case also with 

 human cells which inherited virus pattern from parent. The virus 

 particles thus released from the cells may be scattered about through- 

 out the body to cause systemic infections. 



The persistent immunity must be based on the very powerful 

 structure of the virus in question, because the host protoplasm will 

 recover its normal structure by overcoming the virus structure unless 



