Most Helpful Girl
When you see something gross, your brain subconsciously believes that there are germs on the environment from that gross thing. So you lose your appetite so that you don't eat at all for a while just so that you don't fall sick.
This is also why people vomit when they see others vomit. Your brain gets a signal that the food you've eaten must have contained something that is harmful and you need to get it out. It's called positive feedback. Our bio teacher gave us a very detailed lecture on this topic.