The direct roleblock and the knife attack being separate actions only makes sense if one of them is Mafia and the other is Third Party.
Which could make sense, as the implied idea with the infection is that whoever has that picked Shade because he was going to visit someone, while the roleblock was to specifically stop Shade from doing what he said he was going to do.
In this setup I would guess the infection is coming from the Third Party, and the roleblock is Mafia.