The mortality rate is less than smallpox, but it's still extremely high. Some people don't get sick enough with the disease to require hospitalization, but a significant portion do, and many of them require intensive care.
If we're lucky, this illness will spread slowly, because we certainly do not have enough hospital beds, and certainly not in the intensive care units, to handle millions of cases.
from
http://www.hkupasteur.hku.hk/hkuip/SARS.html "As on April 12th it cannot be told whether the disease is spreading in a linear or in an exponential fashion. The increase in the recent days is a matter of great concern, since it suggests exponential growth. If the total number of cases reached 5,000 by the end of the month, this would suggest that the disease may have come out of control. Mathematicians at the Department of HKU are investigating the possibility that the disease would result from mutations derived from a rather innocuous highly contagious disease (with oral-faecal transmission route) that would have changed its tropism by mutation. This has already been observed in livestock in the past few years, with coronaviruses in particular."