But how do bacteria fight against tumours? The answer lies in the environment the tumour creates during its growth. A tumour, by definition, is a mass of cells that have divided in an uncontrolled, abnormal manner without respecting the normal boundaries. Bacteria are known to modulate our immune defence system and strengthen it to fight against tumour cells. Bacteria activate our immune systems and provide an environment for the production of T cells, a type of lymphocyte, that displays greater ability to specifically recognize and act against tumour cells.