Why must the Torah write that if it spread, he is Tamei? We can infer this from verse 34!
: One might have thought that verse 34 discusses when there is no black hair, and we infer in such a case that it is Tamei if it spread. Our verse teaches that in every case, it is Tamei if it spread.
It seems that this is his intent; perhaps some words are missing. (PF)