I would consider health to be the biological status of an individual and its deviation both positively and negatively from homeostasis. Health can be affected by personal choice, culture, and environment. To me, illness is a negative deviation from homeostasis that is experienced and perceived (but not limited to) the individual with the illness. The illness should be preventable/manageable and relatively uncommon.
These definitions are strongly influenced by my family, my personal experience, and my education. I believe my definition is very biologically based and in that way, limited to my familiarity with a biological definition of health. My definition of illness is particularly influenced by my experiences and observations with my family and their bouts with “illness”.
In considering migraines an “illness”, it is sometimes difficult to differentiate someone else’s “migraine” from a bad headache. To me, a headache is too common to be considered an illness and may go away on its own. For this reason, it is difficult to immediately consider a migraine an “illness” since the observer cannot know the degree of pain experienced by the individual with the migraine. However, a family member of mine is a frequent sufferer of migraines and not only has trouble parting with them with the use of strong medications, but has obvious outward deviations from what I would consider homeostasis. The observer can almost sense, imagine, or relate to the degree of pain expressed by the individual with the migraine. I would therefore, consider migraines an illness.
Spirit possession was somewhat difficult for me to consider an illness by my own definition but I believe my opinion is very biased due to my social and cultural context. It is nothing that I believe is possible, but the point of view of the “possessed” or the view of someone within that cultural context is also important to consider. They may vehemently believe what they are experiencing is an illness caused by spirit possession, but by my definition, may simply be explained by something like the flu. I would say that spirit possession itself, is not an illness, but that the biological manifestation of the “possession” could be an illness.