6 THE PHYSICS OF VIRUSES 



be deduced. Thus T--^ phage has three linkage groups, and at 

 least 15 distinguishable genes. This will be discussed in more 

 detail later. 



Mutual Interference of Viruses 



Two wholly unrelated viruses, which operate in two different 

 classes of cell, seem to be capable of separate multijilication, so 

 that a joint infection of a large organism containing both ty])es 

 of cells will yield some of each in the end. However, if two 

 viruses can grow in the same cell, they interfere with one 

 another (unless they are very closely related) so that from a 

 mixed strain only one will develop. However, in a multicellular 

 organism there may be "victory" by one virus in one cell and 

 the other in the other cells. Thus a mixed strain of two viruses 

 may stay mixed after an infection, but the properties of the two 

 are likely to be changed. Very closely related strains, such as 

 two mutants of the same virus, will give a mixed yield. This 

 a])i)lies to very similar viruses in some cases, such as T-'i and 

 T-4 E. coll bacteriophage. T-1 and T-7, which are morpho- 

 logically distinct, always mutually interfere. 



Virus Attenuation after Multiple Passage 



A very important feature of applied virus research is the 

 production of attenuated strains which have lost their virulence 

 for one host, and maintain the ability to multiply harmlessly. 

 Such a virus can produce full immunological protection against 

 a later invasion by a dangerous virus and is obviously a very 

 desirable agent in disease control. 



The procedure used is to try to find secondary hosts, notably 

 chick embryos, which will survive enough to permit serial 

 passages. This is kept up, and every so often the virus is tried in 

 the original host. Very remarkable results are found. In some 

 cases, after several hundred passages, the desired strain is pro- 

 duced. In other cases, although a virus which is not pathogenic 

 in the new host can be developed, a full return to virulence after 

 one passage in the old host is observed. 



It is hard to be very definite as to what this process actually 



