The basic explanation for the hemocytes is that once the virus has found and devoured all malignant cells, they start devouring healthy cells for fuel, and replacing them with more virus cells, which, despite leading to sensitivity to sunlight and hemophilia of the worst kind, also strengthen the body beyond imagination. It also allows the skin and bones to reinforce themselves at will, making them, when at their best, bulletproof. Only massive blood loss, severe heart damage, and sunlight can kill them, and even then, if exposed to more blood, the bodies may even reanimate. Hell, the vamps even feed on each other to get stronger. And Protosevich's script throws every single one of these elements into the mix during the massive fights that occur outside Neville's house. The vamps are ready for war every night with Neville, complete with chanting and demonic drumming. There's fire, bullets, bombs, impalements, houses being ripped apart, bodies being run over. In other words: utter chaos. So, if a director not known for their dramatic strengths takes the film, be confident that the action will at least blow you out of your socks.