I seem to remember a medical condition called the cruds. Maybe wwII vintage. Maybe not. The overall symptoms were -you just felt cruddy! I’ve searched around for verification and have found it to be the name of a band. where did the word come from and what is its current definition?
I don’t think I can answer your question, but my step-father used to tell me I had “the crud” when I was a kid and caught a cold or the flu. He was in WWII, and perhaps that’s where he picked up the term. My brother further refined it to “the creeping crud”.
Can I assume your brother is older?
I heard this expression from my grandmother (born in the 1930s), but the way she used it made it sound like a venereal disease, or something along the lines of lice or crabs-- something shameful and nasty.
She expressed shock one time while watching a variety comedy act in which one of the characters claimed to be afflicted by “the creeping crud”. She asked me, “Can they really say that on TV?”