Well, pathogens hav specific components that help them to combat host immune system example : capsule, cell wall,production of enzymes , molecular mimicry etc. These components are coded by genes of the pathegen and such genes are called as virulent genes. The degree of the pathogenicity is called as virulence, i hope its clear.

Thanks for help

Knock out the gene responsible for the production of the virulent factor.

try with siRNA. with this you can study multiple genes at a time.

Thanks