Journal entries indicate that Stolls was working on an 'anti-thing virus' along with Ryan and Cohen. Stolls, apparently a dedicated employee, spent an entire night doing intensive research into their 'current specimens'.
Discovered by Blake in the weapons laboratory lobby, he and his equally paranoid colleague Ryan hold each other at gunpoint. Another man, Cohen, had been allowed by Ryan to fetch medical supplies and had not returned, much to Stolls' concern. With Blake's intervention, Ryan is revealed to be an imposter and Stolls is indeed human. Aiding Blake as far as the black weapons storage area, Stolls also succumbs to the infection and reveals himself to be a Thing.