::::In my opinion, "Norovirus, better known as the winter vomiting bug, is the most common stomach bug in the UK" is a definition, but not a very good one, but your one doesn't even reach that level. "Norovirus is a very contagious virus that can infect anyone" isn't a definition either; for example, is it saying that any contagious virus that can infect anyone is a norovirus? It's overbroad.