In the absence of a contractual agreement saying otherwise, the lawsuit would probably just be subject to normal rules of tort liability.
In that case, the contractor would probably lose his case unless he could prove that the one worker infected another through an act of negligence or could otherwise prove that the infected worker knew he was infected and posed a risk to others.
In the basic negligence situation, the contractor would likely rely on the general duty we all have to avoid creating unreasonable risks of injury to third parties, and he would need to argue that Worker A somehow breached that duty. Coming to work knowing you're infected would almost certainly satisfy that standard, but it might be enough to simply show that Worker A was at a large gathering of unmasked people whose vaccination status was unknown. From there, he would also need to prove that breaching that duty caused him some injury, presumably by infecting Workers B through M, causing a work slowdown, causing missed deadlines, causing late fees, etc.
The contractor might also pursue a claim for reckless, rather than negligent, conduct, if Worker A knew he was infected and came to work just the same. Or he might pursue an intentional tort claim if there was some reason to believe that Worker A was actually trying to get other people sick, as opposed to just ignoring the fact that such a risk existed.
As I understand it, several states have also passed laws limiting liability for exposure to COVID in the workplace, so it's possible that none of these claims would be viable, no matter how strong the evidence.