I am curious what makes you believe the immune system is not capable fighting viruses, bacteria and so on. It is exactly what it does 24/7 we are under attack.

If it comes to the newest mutation of the coronavirus it's proved most people do not get sick (being positive is not the same as having covid-19) and out of those getting ill only 2% ends up on the IC. Sounds to me the body is doing a great job.
Ventilating our homes several hours is what we should do, go out, fresh air.