Are flu's always caused by influenza virii? What's the difference between influenza virii (eg. causing Swine and avian flu) and coronavirus (for example causing SARS) in terms of nature and symptoms they cause in people? Why wasn't SARS called flu?
With SARS, you have a modest 92% chance of survival. That's better than the regular flu that kills tens of thousands of people per year! 
It's just harder to breathe with SARS.