Yes, in versus mode you can attack the other infected. However you can only use your secondary attack on them and unless you are the Tank or attacking the boomer* it takes a long time to kill them. Also, this means you are working AGAINST your team and in many cases would get you kicked from a game. * The boomer only has 50 HP, if a hunter or smoker slashes them twice the boomers health will be low enough that they will explode on the first melee strike from a survivor, covering them with bile.