"Strep" is short for a genus of Gram-positive cocci called Streptococcus. Usually, the most common bacteria causing a throat infection (i.e. "strep" throat) is Streptococcus pyogenes.
The "other woman" in a case of adultery is sometimes call the "mistress". The corresponding term for a male who becomes involved with a married woman was historically co-respondent, used in...