I think it's the uncertainty - that it could happen anywhere, any time. The last bug I had, last February, it hit me hard and sudden while watching my sister's swimming lesson. It wasn't the end of the world but obviously it wasn't fun. The worst part of the ordeal is honestly the nausea for me. The actual vomiting part isn't so bad, although with that bug I didn't end up being sick, everything but. I had thrown up a few months before for the first time in thirteen years. The vomiting sucks but like I said, the nausea for me is the worst part of it all. Feeling so horrendous and not being able to stop. As soon as I'd thrown up a couple of times, I felt so much better, and the second time I wasn't even afraid to do it.
I definitely do think it's a control thing for a lot of people - not that we're necessarily control freaks but the lack of control we have over vomit can be scary. It's such a common phobia yet very few people talk about it or seek help because they feel so embarrassed. Since I've started being more open about my phobia, and in-depth about how much it affects me, I've discovered a lot of people who feel the same way. Some people aren't plagued by thoughts constantly, but when presented with a situation involving vomit, react exactly like an emetophobe. That used to be me - only afraid when presented with a situation, as opposed to living in fear.