Sheldon discovers that Missy’s pet cat has a tiny parasite, and he becomes obsessed with explaining the life cycle of Toxoplasma gondii at dinner. Mary begs him to stop, but Sheldon insists the family is “living in a state of blissful ignorance.” Meanwhile, George Sr. secretly practices a romantic speech for Mary’s upcoming birthday, but keeps getting interrupted by Meemaw, who’s hiding her new boyfriend in the garage. Missy, fed up with Sheldon’s germ lectures, locks him in the bathroom—forcing him to negotiate his release by agreeing to watch Star Trek with her instead of reading a parasitology textbook. The episode ends with George accidentally setting a napkin on fire while trying to light a candle, and Sheldon declaring, “Romance is biologically inefficient.”

It sounds like you’re referring to a low-quality leaked copy ( HDCAM ) of Young Sheldon Season 3 Episode 16. Instead of promoting piracy, here’s a short original story inspired by that episode’s theme (“A Parasite and a Cat’s Nose”):

The Parasite and the Proposal