Let's examine the time when you are sick and feeling ill, and look at the effects on the immune system.

Virus infection or bacterial infection.

This is when the immune system is under attack and is being over-whelmed. The situation is likened to a war within your body. Invaders in the form of viruses are causing illness and suffering.

Cancer growth. Cancer cells thriving and the war within the body is very real. The cells are competing for food and energy. The immune system may have started off already weaken (by aging, or by being constantly active with chronic infections or exposure to toxic, carcinogenic substances, stresses etc). The immune system is further weakened when it is losing its fight to cancer.

Many chemotherapy treatments further weaken the immune system.