On campaign, the special infected usually target the survivor that has most advanced on the map (aka front line), some can attack from behind, not necessary a rule.

On versus mode, players can attack as they want.

Server owners can change very little on the game AI, not much to be honest..
But they can do everything else, damage reduction (or completely immune), spawn things, change life, etc..
Basically everything can be done with mods.

ps: if is coop, pay attention to infected noise and stuff, usually bots are very predictable on their attacks.

How come a jockey or hunter will jump over the server host and attack me?? Like the host not even there!! Something’s fishy