Are we in deep depression now?

Immunity. Same for every virus.

Polio, for example, is a virus. Has it taken hold?

?? Why not? Stage 2 means curbside pickup, some companies partially reopen. If it works and infection rates/deaths continue to decline, then we continue. If not, we go back to a stricter level.
I don't know, Bill. Herd immunity sounds like a better gamble. Because of some physical conditions, I am a high-risk individual, yet am nearing the point of throwing up my hands and letting go.
Group exposure to a contagion.
No, that's group exposure to a contagion.

Herd immunity is an immunologic state of a population where enough people are immune that the disease can not easily propagate, and thus new infections decline. You get this in modern society by immunizing everyone or almost everyone. It's how we ended polio, for example.

You can also let the contagion run wild and kill a significant fraction of the population. After all the at-risk people are dead, and after the healthier people have almost all had the disease, then you also get herd immunity. It's how we became immune to (for example) bubonic plauge. After most people got it, and a significant fraction of the population died, enough of the survivors were immune that the disease could no longer propagate.

So we could certainly go that route - but keep in mind that means that the at-risk people (like you) die and the rest of us acquire immunity.