"Those who control the Quarantine Protocols, control the entire Quarantine Zone" - Sanisec CEO, Robert Kathowane

A zombie virus spreads through America and the whole world. Governments collapse. At first the Federal Government attempts to establish a network of QZs (Quarantine Zones), but due to these zones being scattered, lack of coordination and a pressure from both the infected and lawless factions, the US Special Unified Command (USSUC), which meant to hold the QZs together collapses. Some QZs endure, others fall to rebels, insurgencies, raids or the infected.

In Wichita, Kansas, a pretty successful QZ was established, controlling Sedgwick, Harvey and Butler Counties, the Federal forces held on pretty well. But over time the Bunker Dilemma showed up (when the guards and the millionaire get into his bunker during an apocalypse, the guards will turn on the millionaire as money no longer is important) but instead of guns, it's medicine. Private Medical firm in the QZ, Sanifax (former Wichita wing of Wesley Healthcare), overpowers the federal troops as their services began to dictate quarantine measures and hence, governance. Sanifax renames itself to SaniSec (Sanitation + Security) and renames Wichita Quarantine Zone into the Sanisec Governorate of Wichita.

Sanisec swallows the Federal troops, turning itself into a sort of PMC (think CP2077's Trauma Team but without the futuristic tech and not doing it for the money, cuz money isn't important in post-apocalypse). Inside the SGW (the 3 aforementioned counties) everyone is under strict, martial law style governance but everyone gets basic healthcare package (want gold or premium? pay up), food, clean water, security. They also operate in nearby towns BUT only rescue those who pay for coverage.

They accept strangers regardless of wealth (as cheap labour is still valued) but the entrance checkups are strict, people are held up in cages for several days and forced to undergo multi-phased tests.

Those who commit serious crimes against the "Public Order and Wellbeing" are put into forced labor brigades but also are used for human experiments as Sanisec wants to "employ all means necessary in the rapid development and deployment of a vaccine" even if it means doing very inhumane things in secret laboratories.

While the rest of society lives in Martial Law like conditions (curfews, rationed electricity, total disarmament (bullets are a finite amount, so all ammo must be surrendered to the armer forces), regular mandatory medical checkups, etc.) once you learn the rhythm of life, you're free. You can trade, barter, get the job you want/need, and exist without the feeling of an overbearing Big Brother (it's too resource-intensive and quite frankly, unnecessary as Kathowane is not ideological).

This is similar to the "Russian Triune" structure: Velikorossiya (Russia), Malorossiya (Ukraine) and Belorossiya (Belarus) (and Siberian Cossack settler colonies) during the Russian Empire. Just Mirrored onto Japan and put into modern day.