Here are the potentially relevant details:
1. DS has food allergies. The hosts served appetizers and a home-cooked meal from scratch, and I am fairly certain they were able to avoid his allergens. The menu was cleared in advance by me, and we discussed making subs for margarine, butter, etc. DS has never been known to react to cross-contamination. There was cheese and bread served as appetizers, but we don't keep these things out of our house, although DS did TOUCH the cheese. Sad that he didn't know brie to id it as cheese (if he knew it was cheese, he wouldn't have touched it).
1a. The menu consisted primarily of foods that DS has had before, especially the foods that he choose to eat. (corn chips, lentil soup, rice with raisins and carrots, lamb, coconut milk ice cream, spices)
2. The hostess wasn't feeling well that day and evening. Her husband cooked the dinner. She did spend time being close with DS, playing trains with him on the floor after dinner. I saw her lie down, as if she REALLY wasn't feeling well. Her nose was red through-out the evening, and she mentioned that her stomach wasn't feeling well. (So, I'm not sure if her illness was primarily respiratory or if it was a stomach bug.)
3. They had a dog which was very child-friendly. DS spent a lot of time with it, loving it up, etc. They shared a corn chip. DS was carrying the corn chip, dog jumped up to get it (and did touch it with her mouth), and DS promptly ate the chip. DS has not been known to be allergic to dogs, but we did give him Claritin before we went to their house, and his nose was a little stuffy when we left. DS has never had so much dog time before in his life. Never previously swapped saliva with a dog, as far as we know.
4. DS fell asleep on the way home and we put him promptly to bed when we got home. He had nothing to eat after he left their house.
5. He did not have a fever or otherwise appear to be feeling poorly. He said his stomach felt bad after he threw up, but he went back to sleep.
6. DS is breast fed, but he had not nursed since early that morning when he threw up at midnight. (So, it's not likely that he was reacting to something I ate or my reaction to something that I ate. And, just so you know, we eat the same diet, and I avoid his allergens.)
Updated to add:
He ate breakfast (hash browns & sausage), but refused lunch. Threw up again at 2pm. Felt fine afterward; no fever and no illness behavior. Ate a fine dinner.