Too Vague. There are many, many types of diseases. Some caused by infection, some by body stresses, some emotional, and some by genetic defects. This question is too broad.
Using the site. You can get the most out of this public site by providing basic background information about a problem and asking a related specific question,The vague non-specific question you asked has no specific answer. As such you cannot gain anything from the non specific responses. Why is there air ??????????