The only thing they know for sure is that the virus is spread from person to person through the air. So they are following the age old solution for an airborne epidemic, separate people and the transmission will stop. It is not an elegant solution but given time it will work.
Herd immunity is the solution.
And what do you call "herd immunity"? Everyone who is part of a risk group has to die, so you are able to have a nice life with their money?
A natural herd immunity will come, when we do not find first specialized medicaments and/or a good vaccine. But this herd immunity has to come slow - otherwise the health care systems will have no ressources to help the people. To slow down the number of infections is most important in the moment. When 1 person is infecting less than one other person the pandemy will slowly die. If it are more the pandemy will grow exponentially. It were partially 4-6 people - what will lead without any doubt to a murderous chaos.
That theory is that many younger healthy people will not be as affected or effected at all.
Nevertheless Corona is able to kill everyone. In the group from 35-59 years old Germans for example Corona seems to kill three times more men than women. Do you like to infect all women for to protect men? An absurde idea!
It is reported all the time that many have it........and didn't even know it......
That's possible - nevertheless not "normal" - whatever is normal in context of this virus, which is also able to mutate, what's a higher risk, if no one fights against this virus.
Since they have had it, they are saying that they can't spread it anymore....................
I don't know, who your "they" is. An unknown population, which is doing whatever you think it will do, because it lives only in your thoughts?
So the math that way is if a large portion of the healthy population get over it......then they will reduce the threat because they can't get it or give it away again.
Your "math" is when everyone is a lazybone, who doesn't take care, then "an invisible hand" will come and will solve your own personal problems - but latest in the moment when you will not be able to breathe any longer and you are going to die in panic then your last thoughts could perhaps be:
"Send all assholes to hell who killed me with their wrong ideas." - what would only make your own situation much more worse.
Sweden and Brazil are the test subjects right now...........we shall see.
The people of Sweden are honest, disciplined, social and empathic. They don't need concrete laws and orders. They need only good information. I do not think they are happy with their way, when they take a look at Germany. 50% less victims in Germany - compared with Sweden and relativelly seen to the number of inhabitants. Perhaps the way of Sweden is not wrong - but it is an experiment - far away from the situation in Brazil or in the USA. Unfortunatelly they seem not to have good data for this experiment. They seem to fly blind. But Sweden is able to react fast and consequent - much faster and much more consequent than most other nations in the world. They are able to correct a wrong way the moment they see it.
And Bolsenaro had damaged more than enough meanwhile. Fortunatelly he was stopped from the laws of Brazil. In former times such an irrational and hateful man had to be hanged, when such men became a serios threat for the survival of the own people and all mankind.