That possibility looks reasonably slim to me
That is one thing that contact tracing does for you.
If you have some cases of SARS which are not caught because they do not advance beyond something like a cold, I would still expect to see them infect others. And you would see that in the form of people showing up with SARS and no explainable route of transmission. You might not find the intermediate carrier, but you can infer their existence. We have not inferred their existence yet, so I am inclined to think that they aren't there.
This logic assumes that people with mild symptoms can infect others and give them full-blown SARS. That is a big assumption. If they do not infect others, then their existence skews your statistics but it isn't that big a deal. If they infect others but only with a mild form of SARS, that probably means that the bug mutated. Which could actually be good news. A mild form of a disease often can innoculate you against a serious one. (See cowpox vs smallpox.) In which case failure to spot that does not make the epidemic more scary, rather it is a potential opportunity lost...
Cheers,
Ben
"good ideas and bad code build communities, the other three combinations do not"
- [link|http://archives.real-time.com/pipermail/cocoon-devel/2000-October/003023.html|Stefano Mazzocchi]