Illness , both physical & mental, (otherwise known as dis-ease) is typically caused by old trauma and trapped emotion.
When you have emotional issues that just get buried and never actually dealt with to clear away the negative energy associated with it, it can and typically will manifest as illness in the body. It will also keep you from manifesting abundance.
A good percentage of the human population has developed limited belief systems that are not serving them and holding them back from living their best life.
Once you learn how to clear away negative energy & blocked chakras, you will be able to manifest with ease.