Asymptomatic: People could have the virus and not exhibit the symptoms. This virus could already be out there, widespread, and the people we are seeing are the ones reporting to the hospital.
And you thought you just had a head cold, little did you know it was SARS: only at that time it didn't have a name. How many people have had influenza and not reported to the hospital?
At which point should I move to a small island in the Pacific?
This is one of the biggest questions on the planet.
1. If lots of people have the disease and just don't know it, then it is both completely out of the box, and not nearly as nasty in percentage terms as it seems.
2. If very few, but some, people have such mild cases that they do not know they are sick, then we have a disease fully as nasty as we see in Hong Kong, plus no way to make sure we have all cases in quarantine. This is the worst case scenario.
3. If ALL patients get sick enough to know they are sick, and they are not contagious until they are visibly sick, then we have a built-in way to slow down its spread. This is probably the best case.
The evidence thus far is scanty, but it does not seem to support the "many mild cases" scenario. Aside from the fact that the U.S. CDC and the WHO have indicated they do not think that is the case, the best "laboratory" example we have does not support it either. Prince of Wales Hospital had very few mild cases, and a very controlled group people to monitor/check.
On a larger scale, we are not seeing community transmission from unknown carriers anywhere in the world except in Hong Kong and China. Even in Hong Kong, community transmission from unknown sources seems to be rare. Hence, the evidence thus far does not support the "many mild cases" postulate.