Your immune system is perfect already, until you over work it
Fasting forces the body to burn off the bacteria in your body because that's all it has to use as energy, so you clean out your system by fasting, so now your immune system can use 100 percent of its resources to fight off anything trying to infect you
so after eating a life time of bullshyt, smoking, etc, your immune system is over worked so when some new shyt attacks you, it doesn't have the ability to protect you because its too busy dealing with all the bullshyt you put into your body for years and years
that's why you have to fast and clean out your system