The natural history of virus mutations is increasing transmissibility with decreasing virulence. Omicron mutated it's spike protein in a way that greatly increased infectivity in the upper respiratory tract but severely reduced it's ability to infect lung tissue. Hence all the URI symptoms with relatively minimal pulmonary problems.
Back on topic, Envision's debt is mostly the result of the leveraged buyout by KKR. You may remember leveraged buyouts from such success stories as Toys R Us. Ideally, you find an industry that has steady consistent profit but may have some vulnerabilities due to changing market conditions. You then acquire a business in that industry that has brand recognition. Since that can be expensive and risky, a significant portion (70-80%) of the purchase cost is actually a loan secured by the collateral of the company you just bought. Pump a bunch of cash into expanding the business (ie acquiring contracts) then take it public after showing explosive growth. Reap the windfall from the IPO, wait for the artificially pumped up growth to subside back to baseline levels, then repeat. If everything works as planned, PE acts as booster rockets for the CMG's growth. The problem is as the debt keeps increasing, the percentage of the business's profits spent servicing that debt rise. You're turning something that was safe and reliable into something that is more profitable and significantly more fragile. Which leads to things like APP (recapitalized in 2017 by Brown Brothers Harriman) forcing its private contractors to give them an involuntary loan during the pandemic so they didn't default in the summer of 2020.