Diseases of the auto-immune system have become more and more widespread nowadays.
The basic point about these diseases, which is also their mystery, is that cells begin to treat certain parts of the body as foreign bodies.
Every living organism has something that defines it; the body knows itself.
When a part of the body is hurt or wounded, the body always feels: this is I, whatever is now causing me pain is me.
In the auto-immune diseases, the "I" ceases to identify itself; the picture of the "I" becomes partial, stilted, reduced.
And then, some of the cells begin to do what they would do against any foreign body: they try to eject it.
They become incapable of perceiving the former complex "I."
All of these diseases, which are horrible diseases, are not caused by a germ or a virus; they take place within the body itself.
Like in the case of AIDS, the apparatus that triggers this response can be created spontaneously, or due to external stimuli such as foreign blood, or relations with people with whom we should not have any relations.
But at any rate, the reaction is similar: the body ceases to recognize itself as a single unit, and begins to perceive parts of itself as strangers.
Such a situation is not just scary: it is an encounter with death itself.
So long as the Jewish "I" knows that a Jew is a Jew – however much he may fight with him or be willing to cast him to hell – that is a different, much more intimate and personal case.
It is my own self, a part of my "I." It is like the attitude that exists within a family.
I can be angry with the bad boy, or with the delinquent brother; I can even throw him into jail.
Still, I know that he is "my bone and my flesh" (Genesis 29:14), even if we argue, even if we tear out each other's hair.
So long as he is I, a part of my "I" – the "I" of the individual or of the family – we both can exist.
But this disease – in which the "I" ceases to recognize itself as a unit, a comprehensive whole, and is willing to recognize only certain parts – is a state beyond repair.
Such a situation is not just a partition, it is not just pain: it is death.
–Rabbi Adin Steinsaltz
From an essay, "Achdut – Jewish Unity" by Rabbi Adin Steinsaltz