When Ellis shoots the original zombie in the head with the shot gun, the blood spatter hits him in the face. When he wipes it off he pulls the blood down across his face and mouth. When he speaks you can see the zombie's blood on his teeth. Earlier in the movie, Nightingale tells a character not to put something in his mouth cause it had infected blood on it. If this is the case, the blood that got in Ellis' mouth would have infected him.