Zombies... so many zombie games out there and they are all exactly the same. Try something original! If you want a viral mutation then make up your own symptoms, like for example: a disease which over-stimulates the growth of bones, causing all kinds of gross bony monsters with spikey joints and even carapaces. You could say that it also causes the skull to grow protrusions on the inside, pressing into the brain in certain key places which causes extreme agony, delusions and violent responses, making anyone affected by this disease turn into a bloodthirsty monster.
Just an idea that came to me. Use it if you want. Just please, no more zombie games!