In answering this question, having different points of view or different theoretical foundations, the answer will be different. Western medicine may explain it based on different point of views, from anatomy, microbiology, and even genetic point of view. But, I’m not going to talk about it. I’m only going to elaborate more from the aspect of Traditional Chinese Medicine and soul.
The ‘Theory on Three Categories of Etiologic Factors’ 三因学说 SanYin XueShuo, of Traditional Chinese Medicine, already have a very good explanation and interpretation on the reasons for happening of diseases. The ‘Theory of Three Categories of Etiologic Factors’ are the internal factor, external factor, and non internal cum external factors. Just like the occurrences and developments of any other things, diseases also need the simultaneous existence and interaction of both internal and external factors, in order to generate diseases. It can also be understood as diseases are derived from interaction of YinYang. This is because, it is impossible to have “solitary Yin“ “孤阴” and “independence Yang“ “独阳” in this world. The happening of any kind of disease is not accidental. As in “The Yellow Emperor’s Inner Canon” 《黄帝内经》 <<HuangDi NeiJing>> stated, “With accumulation of exogenous pathogens, the vital-energy, Qi is weak.” “邪之所腠，其气必虚” and “Keeping sufficient vital-energy, Qi in body; exogenous pathogens cannot invade body” “正气存内，邪不可干”.
With same external environment and conditions, why some people fall sick, while some people do not? t The key point lies within the sick people, that are ‘deficiency in vital energy, Qi‘ “气虚”; and instead healthy people have ’sufficient vital-energy, Qi in body’ “正气存内”. Therefore, the highest realm in Traditional Chinese Medicine is not about treating diseases, but prevention and health care. That is “Do not take treatment seriously, just only when fall sick; but take it seriously before sick” “不治已病治未病”. If prevention and health care are well carried, then where would disease come from?
Life is a soul in a body, and it’s alive; human is the mixture of body and soul. What I mentioned here, ’soul’ is similar to ’spirit’ mentioned in Traditional Chinese Medicine. Human’s body is being controlled by soul, including behavior and thought. So, I think that, many diseases happened as what human ‘think’ of them.
No one really wants to fall sick! Human’s behavior and thought are controlled by soul. Each and every act of human action; judgement and thought of each and every thing; also have a positive or negative message, or both messages at the same time. This is also one Yin and one Yang. Human and nature is a harmonious unity, where Traditional Chinese Medicine named it as “Unity of human and nature”. Each issue of a message or known as brain wave from human body, will affect the surrounding people and environment. While for this kind of effect, often is the negative effect that come faster. This is because universe receives negative messages first when receiving messages. Thus, when universe gives feed back, naturally it is also negative effect.
For example, when a person continuously work for long hours, the body will send out a message – ‘I’m tires.’ What to do when feel tired? I need rest! Then, how to have a break and rest? I would like to fall sick. And this happens where a person is totally unaware of. It is entirely controlled by soul subconsciously. Moreover, if always having negative thinking, such as ‘Will I have the same disease with him?’ and ‘What can I do, if I also have the same disease?’ when saw a patient, and it might ended having the same disease. This is because you are always sending out the same negative messages.
Since disease is from thought, then it can totally be treated by ‘thought’. This is so-called psychotherapy treatment! Traditional Chinese Medicine called it as ’spirit healing’. It is as stated in ”The Yellow Emperor’s Inner Canon” 《黄帝内经》 <HuangDi NeiJing – YiQing BianQi>>, ‘Those with spirit will be prosperous; while those without it, will perish.’ “得神者昌，失神者亡”