18 AN INTRODUCTION TO THE STUDY OF VIRUSES 
continued passage of potato virus X through successive tobacco plants. 
This frequently results in an increase in the virulence of the disease. 
Since, however, this virus commonly occurs in mixtures of strains, 
differing in virulence, it is probable that during the progressive 
passages a more virulent strain is selected out. 
It sometimes happens that passage of potato virus X through, or 
propagation in, an unusual plant host results in the production either 
of a more severe or a much milder virus when returned to the original 
host. This type of phenomenon can be explained on the grounds that 
the unusual host is either more or less favourable to the multiplication 
of the various strains. 

