This question does NOT have an answer, but I so very much want to know

Supposedly he gets tested every day. Who knows?

remember that there was one Survivor in the small town initially infected...

Both analogs of Tangerine Idi Amin.

A baby who was colicky and crying and an old guy who drank Sterno.

Both of their blood chemistry was off, so they didn't get the pathogen.

It's because he lacks a brain.

Your guess is as good as mine.

Scans for me

but I've opined whether he gets treatments unavailable to the ordinary citizen. Bunker Boi isn't exactly known for his courage where his precious hide is concerned, and it's suspicious that he's going around like he's invincible. My guess: he's been getting antibody injections since early on.

“Giving serum from newly recovered patients is a stone-age approach, but historically it has worked,” said Jeffrey P. Henderson, MD, PhD, an associate professor of medicine, infectious diseases, and of molecular microbiology at Washington University School of Medicine in St. Louis. “This is how we used to prevent and treat viral infections like measles, mumps, polio and influenza, but once vaccines were developed, the technique understandably fell out of favor and many people forgot about it. Until we have specific drugs and vaccines for COVID-19, this approach could save lives.”

So, he probably has a bank of 7 survivors strapped to beds down in his "bunker". One for each day of the week.