Recently I reread a novel set in the early 1900’s in which one character attributed a little boy’s “sores” to, possibly, “bad blood.” Is this a euphemism for some kind of STD, or am I reading too much in? 
They didn’t mean an STD. Medicine was just really primitive back then. “Bad blood” was just short-hand for an infection of some sort even though they had no idea what it was or how best to treat it. They had all kinds of weird quasi-medical terms back then. “Consumption” was the term they used for tuberculosis for example.
Which book are you referring to?
“Bad blood” was a term used for syphilis, but in this context they might have meant some other pathogen.
Here’s a neat list of old-time medical terms. My favorites include “mortification” (infection) and “softening of the brain” (for insanity). Their definition of “brain fever” is puzzling though - in the Sherlock Holmes stories, characters were always coming down with “brain fever”, a mysterious stress/anxiety/depression event which prostrated its victims for weeks but from which they eventually recovered.
By the nineteen hundreds, bloodletting was beginning to fall into disfavor. However, a good portion of the population still believed that bleeding a person was either proper preventative medicine, or could actually treat disease.
~VOW