[SauronOfMordor]

Several things strike me about SARS:

1. It has a high death rate. In Hong Kong, Canada, and other countries, we're seeing death rates of around 18% (when you look at deaths versus recoveries, excluding people still sick). This is in the same league as smallpox
2. Black Plague, while having a higher death rate (50%) is spread by rats and fleas, which makes it a non-issue in the US outside of certain high-poverty areas. Smallpox is not contagious in it's initial stages, and the ill person will be bedridden by the time the contagious stage is reached, making isolation easy. SARS is an airborne disease, and seems highly communicatable
3. SARS mutates, making a vaccine difficult to produce. It also means that those that survive one SARS illness, can get sick AGAIN a few months later from a mutated version (much like getting a cold does not immunize us from colds)

The combination of these factors has the potential to be very ugly. You would have difficulty bioengineering an uglier bug if you tried

[aristeides]

Black Plague does occur sporadically out west in this country, because of infected wild rodents there. But, with modern treatment, it has nothing like the old death rate. If it's caught in time, it's 100% curable with antibiotics. You only die if it's not caught in time, which is usually the result of misdiagnosis.

[Sacajaweau]

It may have a high death rate but considering the population of Hong Cong is 6.8 million people, the rate is miniscule per overall population.