I think I've told you all I remember but in case I forgot something important or it benefits someone else too:
sudden onset for us. DD had a single nebulizer treatment for a cough and turned into a different child immediately. Screaming, crying, shaking, severe separation anxiety. She could not go to school because she started eloping. She could not be left alone because she was expressing suicidal thoughts (at 7!). I had to sleep in her bed for weeks because she could not control her body. Lots of tics. Lots of obsessive and intrusive thoughts. Lots of meltdowns. It was like living with someone in the throes of an epic OCD episode but it went on for weeks.
Her pedi diagnosed in through a strep titer -- her strep tests were negative. She was on an antibiotic for a month, and it didn't work at all for about 3 weeks (dr told us this might happen). Then it did, and she recovered, but it changed her and she really hasn't been the same kid since (she's 15 now).
"Hello babies. Welcome to Earth. It's hot in the summer and cold in the winter. It's round and wet and crowded. On the outside, babies, you've got a hundred years here. There's only one rule that I know of, babies-"God damn it, you've got to be kind.”
Neither I nor my son have ever formally been diagnosed, but I have a pretty firm feeling that DS at least is impacted. We've seen very PANDA-like symptoms after every strep episode he's ever had, and while he has ADHD and is anxious normally, he doesn't have OCD. I DO have OCD, and my symptoms are much worse when I'm sick. My childhood onset of OCD (prior to 9) was preceded by almost constant cases of strep. I'm still prone to strep as an adult. I never pursued it with DS because it seemed exhausting, honestly.