Statistically the entire population has some form of immunity to most viruses. Even the common flu you are somewhat resistant to, especially if you had a flu shot for that year. Even if there were to be a giant "measles outbreak" today, the vast majority of folks have been vaccinated and have little to worry about. Perhaps a booster is needed, but they can administer vaccines immediate and people have the antibodies to fight it already.

The difference here with COVID is that there appears to be little-to-no immunity to it. Some people can fight off with mild symptoms, some do not. Timing is a huge problem. When the general population gets the same illness at the same time, which can greatly consume machines which are not plentiful [ventilators], the system reaches capacity extremely fast. Even then, those people are spending 14-21days on a vent, rather than the typical 3-4 days. For every severe patient that exceeds the capacity of local facilities is pretty much a 1-for-1 death. People go from staying at home with a fever to dead within 12-48hours.

That said, this is the same mindset that took Italy by storm. Italy currently has a 12.3% death rate. [that's a lot]. 15,887 deaths for 128,948 cases. I understand in the grand scheme of things, the numbers aren't astronomical, but it is a pretty ruthless illness that is extremely contagious in close quarters.

The whole goal of this exercise is to make it seem like an over-reaction, that is ideal! People get back to their lives, people recover, life is good. It's quickly reaching a point to where it's not an over-reaction, people are not doing what they should be doing and will continue to make things worse. States are being left to fend for themselves and Governors, many of which have never dealt with a situation remotely to this magnitude are left to their gut feeling for guidance.

I feel like the 100k-240k death projections for the US is extremely low, but we shall see.