In short, the 'strain' mutates on multiple occasions, seemingly at random, but each time in rapid response to the prevailing conditions under which it finds itself;
It starts off as an infectious agent, rapidly killings its hosts but finding more and more people to infect.
When there are no more people to infect, it starts to eat synthetic polymers with very similar properties to human skin.
“Well, no rubber was used in this airplane. It was all a synthetic plastic compound. Newly developed by Ancro; they’re quite proud of it. It’s a polymer that has some of the same characteristics as human tissue. Very flexible, lots of applications.”
After it runs out of polymer to eat, it moves to rubber, a substance very similar to the polymer, digesting the seals in the facility.
It escapes the (oxygen-rich) facility, presumably by decreasing its size, along with its lethality, which may be related, and goes up into the atmosphere.