Someone that pays better attention than I do probably already figured this out but MC infected himself on the 2nd day there. When he jerked himself off after touching the blood stuff on the ground.
Also, he cleaned that stuff up on the 1st day, but it came back. It's leaking from the sink I think.
And they've also swam in the lake. Which is where I think the stuff is coming from before getting into the pipes. So I'd bet both MC and Claudia are already infected, it just shown with MC sooner because he got exposed directly to it first.
When he felt pain in his hand, thinking it was a cut, but he had no wounds. That was the infection spreading. It is in his hand (from touching the blood) and his dick (from jerking himself off). That also means it's in Claudia's...you know.