If a first-born son meets the qualifications for pidyon ha'ben (first born of his mother, born naturally, parents are both Yisroel), what physical health issues would "disqualify" the bechor from needing a pidyon haben. For example, if Chas v'shalom he has a disease or is ch'v blind or ch'v missing an arm would these override the need for a pidyon haben?
There is a very interesting 4-volume set of seforim called Nishmas Avraham by Rabbi Dr Avraham-Sofer Avraham which details medical and health based halachos. They are all available on Hebrewbooks.org - Vol, 1, 2, 3 & 4)
In the cheilek on Yoreh Deah, siman 305 it details the Laws of Pidyon Haben. It is a very compelling read and explores a number of issues. It mentions here - (no. 2 in the Nishmas Avraham) that if a child is born with terminal defects he does not undergo a pidyon haben - even if medical intervention can keep him alive for 30 days. It also mentions a case of a child that is born "ללא קיבה" - "without a stomach" (I think) and Rav Elyashiv paskened that it has the status of a 'tereifa' and if the baby lived beyond 30 days it is still exempt from a pidyon haben.
He writes (in gimmel) according to the Shach that in all cases of a definitive 'tereifa', even if the baby survives a long time after the 30 day period he is still exempt from the pidyon haben. This would apply even in a case of a 'sofeik tereifa' i.e. when there is a doubt if the child has a defect or not. He then goes on to cite a discussion on whether if the child lives beyond a year whether or not he still has the pidyon.
However, it would seem that he concludes:
אבל מי שנולד טריפה מתחלת ברייתו, חייב בפדיון
But one who is born with a defect from the beginning of its creation (i.e. from birth), is obligated in a pidyon.
I assume that means that if it is treatable and he can recover he has the pidyon, but if it is something incredibly serious we don't put the child at risk - like with the case of the baby without a stomach cited above.