The problem lies with the concept of illness.
to consider an example.
i am lactose intolerant. If I eat dairy I get quite unpleasant symptoms - vomiting, bowel issues etc.
this is not considered an illness.
why?
because the vast, vast majority of people are lactose intolerant after weaning. The norm is for babies and toddlers to be able to process lactose and dairy and then it vanishes in childhood.
this is the case with many many mammals, and in most humans except those in Northern Europe.
in Northern Europe due to extensive keeping of cattle milk is drunk by the adult population as well and this is believed to have led to lactose tolerance lasting later and later in life.
however even in Northern Europe it tends to not last a lifetime - many elderly people in England/denmark/germany begin to have problems tolerating lactose.
so because the majority of humans do not tolerate lactose beyond early childhood, the fact that I don’t tolerate it beyond age 20 is not an illness it’s the standard human condition.
what IS outside the norm biologically is societies where milk is drunk into adult hood - I’m an outlier in that society but actually completely normal by the rest of humanity’s standards.
more generally, it can be hard to tell the difference between an illness and a normal variant. People are not all standard and there is massive massive variation in both physical and mental characteristics. Some societies call some variations illness.
the classic example being homosexuality as a mental illness. It’s very very clearly a normal variant in both humans and many other animals. Calling it an illness says more about the society than about the person,
earlier in English history women who had children out of marriage might well be locked up in a mental institution with the diagnosis of “nyphomaniac” when clearly wanting to have sex (or being raped!) is not a mental illness (to today’s society) and it seems obvious now that the real problem was that she had a baby and pissed off some people with power.
illness (and especially mental illness) is not a simple concept