In a war, the stronger party will be victorious. A weak army will soon surrender to its antagonists. The same applies to our bodies. When microorganisms invade the human body, a healthy immune system kicks into action and fights the battle immediately. 

Take, for example, two individuals facing the same influenza virus. One falls seriously ill while the other remains healthy. Although threatened by the same virus, only the individual with the weaker immune system succumbs to the virus. 

Microorganisms like bacteria and viruses are responsible for causing many diseases. Yet they will not be able to wreak havoc on our bodies in the face of a strong immune army, whose job is to defend the body against infections. Most illnesses are, in fact, allowed to flourish by a malfunctioning immune system. 

The human immune system malfunctions when it is weak. Therefore, to facilitate normal functioning, the immune system needs the support of a healthy diet and sufficient rest. In contrast, malnutrition or excessive stress will cause us to fall sick more easily. 

A weak immune system causes the human body to experience more serious, and even dangerous, symptoms when faced with highly infectious diseases such as severe acute respiratory syndrome (SARS), pneumonia, hepatitis, influenza, or tuberculosis. In fact, a weak immune system can even lead to the onset of cancer. Under normal circumstances, the immune cells keep a vigilant watch over every part of the body, always at the ready to kill off any mutated cell that has the potential to develop into cancer. But when the immune system is weakened, it is unable to swiftly identify and destroy cancer cells, resulting in an uncontained spread of cancer cells in the human body.

Cancer accounts for nearly one of every four deaths in the U.S. In 2012, there were an estimated 8.2 million cancer-related deaths and 14.1 million new cancer cases worldwide. Lung, liver, stomach, colorectal, and breast cancers cause the most cancer deaths each year.