Answer :
A division of your immune system called cell mediated immunity is charged with stopping viral infections. The main actors of that are called cytotoxic T cells who, when they encounter a cell that is displaying viral antigens on its surface, binds to the cell and releases toxic substances called perforins and granzymes that destroy and digest the viral infected cell.
I think its because the white blood cells stops the infections before it can infect the body I don't really know
