Hm... calculating from the number of times I see any creature whatsoever vomit and considering the relation between the number of rats I usually have any kind of contact with and the number of non-rat creatures I usually have any kind of contact with, I don't think that's really a valid question to ask. But this blog article and the pages linked from there, especially this essay explain the scientific reason in great depth. Probably not really necessary to know, but a fun read, somehow :-)