Many exoenzymes are also used as virulence factors. Pathogens, both bacterial and fungal, can use exoenzymes as a primary mechanism with which to cause disease. The metabolic activity of the exoenzymes allows the bacterium to invade host organisms by breaking down the host cells' defensive outer layers or by necrotizing body tissues of larger organisms. Many gram-negative bacteria have injectisomes, or flagella-like projections, to directly deliver the virulent exoenzyme into the host cell using a type three secretion system. With either process, pathogens can attack the host cell's structure and function, as well as its nucleic DNA.