There's probably no answer to this, but why does one person gets an illness and why someone related doesn't fascinates me. By way of examples in 1947 I was 2 and my sisters were 3 and 4. The 3 year old 'caught' poliomyelitis. The other 2 of us were unaffected.
My 2 sisters have lived together all their lives. They both retired, early, in the same year. They eat the same foods, and get the same exercise. 5 years ago the older one ' developed' Type 1 diabetes.
I'm convinced a second reaction to tablets I was taking was a step too far for my immune system. The first reaction was painful rashes on my fore-arms; the second was pains in my leg muscles, both known reactions to the drugs I was taking. I'm not interetsed in litigation. Illnesses happen. The medics say it could be caused by an unknown virus, but that seems to be clutching at straws to me. Does anyone really have a clue as to its cause?