To answer this question, a few blog posts would certainly not be enough. It is also quite likely that this is not the aim.
Illness is a condition that has a negative effect on the original structure or function of an organism. (The definition is of course not complete.) The sentence assumes that there is a condition where an organism is stable, balanced, healthy. Disease, on the other hand, is a state of being out of balance. In most cases these are identified by the appearance of signs or symptoms. And these can be further dissected.
Almost all the media lately has been full of stories about viral diseases, their dangers and how to counteract them. At least with one of the supposedly fastest acting shoulder species.
'A university lecturer asked his students why malaria can develop. According to the article, the students had not yet learned about parasites, their answers were logical: dense vegetation, no mosquito nets over the bed, lots of rain, mosquitoes and mosquito bites. Towards the end of the year, they had much more knowledge, so they were able to give more precise answers to the same questions. A parasitic species, Plasmodium, is responsible for malaria. The answer is correct, but the students have still missed a number of other factors. The mosquito breeding site, poverty, and a warm, humid environment all contribute to the development of the disease. One of the most important factors in the parasite causing disease is a weak immune system. The lesson of this example is that there are many factors that contribute to the development of a disease, it is not enough to focus on just one or a few! And this is also true for all other diseases' (Muula, 2016).
The immune system's job is to protect the body from disease. It is our many but very important physical defence system, whose most important function is to protect us from the negative effects of bacteria, viruses, fungi, parasites and more - ( (Newman, 2018)
Its functioning is quite complex and, in my opinion, research is still far from fully understanding it. But one thing is for sure, without it, no organism on this earth has much chance of survival. The strangest thing is that the immune system can act against the very organism in which it is made. And it belongs to that part of our body that, when it's right, we're not really aware of its existence. As long as it's working well, we don't even notice it.
There are techniques that swear by the fact that every element associated with the physical mind, as an ego, must be eradicated because it is the cause of all trouble. Joe Dispenza, through his research with the help of his peers, communicates a lot about the need to fully develop a state of belonging to nowhere, describable by nothing, and this is achieved by a total destruction of the ego.
Others teach that the greatest and most valuable thing is to be aware of the present moment. Followers of "being present", through the power of the mind, encourage us to be in control of our circumstances.
One thing is for sure, while there is more information, technology, religion, etc. available than you could ever need, focusing solely on one side does not create balance. Just as there are several components to falling out of balance - illnesses - there are several components to creating and maintaining balance - or health.
One of the most thought-provoking books I have read on balance is Ikigai by Héctor Garcia and Francesc Mirales. Which is more than a book, of course. Very briefly, the two authors explore why people in places like some islands in Japan are still very active and balanced in their later years.
In my own experience, it is possible to achieve a state of balance. Through mindset, proper communication (energetic too) and developing the right habits. As you can see from the university example above, we need to understand the details, but we also need to understand the parts that we don't fully understand yet. Because the mind is simply not designed for it. You need all the parts to have inner balance. If you reject any part of it, then it is not possible to have balance. And for a healthy life you need balance.
To answer the question in my own post, disease appears when inner balance is out of place. The reason why so many diseases develop is that we pay little attention to this area. There is often so much criticism of this approach that it is seen as a lie, a quackery, by many people, when we can have about as much information about the physical body as we have about the seas around us. We know that there are. We can see how certain parts of it work, but not at all how it works as a whole, its effect on the planet Earth and the organisms that live on it.
I'll end with another favourite thought:
"Most people now have a device in their hands that has a knowledge far beyond that of the previous moon landers and more than 90% of their operators. What about the human body? Have we read and understood its manual?" - Sadhguru
References
Muula, Adamson S. 2016. https://theconversation.com/. https://theconversation.com/what-causes-diseases-why-the-answer-is-complicated-and-contentious-55955. [Online] 2016. April 4.
Newman, Tim. 2018. https://www.medicalnewstoday.com/. https://www.medicalnewstoday.com/articles/320101. [Online] 2018. January 11.