What is happening when we become unwell?

I'm a student of META-Health, which is the art and science of self-healing. It helps us to identify the root cause of physical, psychological or behavioural imbalances. It integrates old wisdom such as how our biology works, with modern day knowledge of how our brain and organs work and communicate together, subjectively, as individuals. META-Health is naturally holistic at its core.
Have you ever wondered why people who are exposed to the same health risks don’t develop the same symptoms?