CONTROL OF VIRUS DISEASES 95 
invasion by a second virus provided the two viruses are closely related 
strains of the same type. Now if, as sometimes happens, one virus 
strain produces a very mild disease whilst the parent or type strain 
gives rise to a severe one, a plant can be first inoculated with the mild 
strain and thus be protected from infection with the severe strain. 
Mild symptomless strains occur in tobacco mosaic virus and in 
potato virus X, and some preliminary attempts at protecting potatoes 
and tomatoes by this means have already been made. 
