Actually a defence system called immune system is present in our body. The immune system possesses specialised cells which are capable of killing the infecting microbes. When microbes enter our body, our immune system becomes active and the cells are sent to kill the infecting microbes. If these cells are successful in killing the microbes, we do not come down with the disease. But this is possible only when the microbes are in small number; large number of microbes cannot be successfully invaded by the immune system. That is why, immune system is very essential for our health.