Cold viruses spread through respiratory particles and stimulate the release of these particles (through coughing and sneezing, for example). Gastrointestinal illnesses spread through feces and vomit, both of which they tend to produce as symptoms.

I very much doubt that this is accidental. It seems selected for; viruses that symptomatically increase their own chances of spreading must be more successful than those that do not. Of course, viruses do contain either DNA or RNA, both capable of propagating, and thus are able to evolve in such a manner.

The puzzling thing, then, is why some viral diseases traditionally caused swift death. It would seem that they deprive themselves of living cellular machinery to produce more virus – what would appear to be a losing proposition.

I still think they’re alive.