If others were sickened and reported to UA, you can expect UA to reach out to you and try to make things right (and obtain a signed release at the same time). If you want to report it, you should send in a short note along with your medical bills and the like. If you are the only person who reports, you can expect a polite but firm report back from UA to the effect that nobody else reported any issues and UA served X hundred of those meals from that batch of curry that day, so must have been something you touched on GCM or the like.

Proving source is a rough road given that you transited at least two highly public places on that day alone, e.g. ORD and GCM and could have bumped into any number of very sick people anywhere. Further, food poisoning can take days and it could have been a slightly old bit of something in a salad from 3 days before.

I am sure that you are disappointed, but you are being a bit quick here to assume that it is UA and not you or some other business or person who fell down on the "bug" issue here.