The problem is that the asymptomatic can spread it widely without realizing they're doing so. The mortality rate is not Ebola high but it's not unsubstantial either. And when you don't have a public health system that covers all, then there will be millions of poor people and illegals that will get it and spread it. The poor are also the ones that can't stay home for two weeks and just ride it out. Those who go to the doctor, and then probably to the emergency room, only when they're in dire straits may spread it to hundreds of people.
The mortality rate is between 2-3%, nearly 15% for people above 70. Of course, the better the health system and precautionary measures taken, the lower the death rate. But let's be optimistic and say we'll have a mortality rate of 2% here. Let's say just 10% of the population contracts the virus. That works out to about 660,000 deaths.