When L.J. Is bitten he manages to suppress becoming a zombie for 1-2 days. When Carlos is bitten by L.J. he is infected so quickly and talks about it "being too late for him" (whereas in the previous film he had 3 hours before getting the antidote). Why is this different now and why is Carlos not still carrying the antidote in him?
When Alice's clone stops the lasergrid from killing the real Alice, there is a shot of the computer screen showing the disintegration of Isaacs, Dr. J. This is a reference to Jason Isaacs, who has a history of appearing in Paul W.S. Anderson movies. Anderson produced this movie, so the circle is complete.