We've all had multiplayer discussions about playing as the infected in survival multiplayer modes and how that could work, but I don't recall any discussion about how the controls would work. If we do play as infected, I yhink it would be a sort of team deathmatch/ infection mode (One player starts out as a clicker and infects the other team getting more survivors on their team, also as clickers). The normal runners would just be very fast and could only melee. You should be able to clearly see everything, now moving the left analog stick will automatically be a jog/run, holding L2 while moving the analog stick is a sprint. Using square to attack the survivors, this could be used in the infection game mode or team deathmatch. (In infection the main infector would play as a clicker, while when they get more players on their team they'd be runners. In deathmatch the infected team would be all runners.)
The clicker is the most interesting of the infected to play as. Clickers are blind, so it would only make sense that your screen would be black. As you push a button to make the clicking sound, the area around you will be outlined with white lines. You won't get too many awe-inspiring visuals as a clicker, which is a downside, considering the visuals are amazing. But you would have an immense amount of health and could get one-hit kills. Making you a big threat, it would feel like being the Tyrannesarous Rex from Jurrasic Park (Big fan of the original trilogy and just saw the new 3D one, save your money it's not that great). This would be an amzing mode to play with friends on, considering you could all have fun over the mic talking and having laughs. (That's at least what I do) Plus, it'd be fun to play with friends on-line late at night, making the gameplay of Infection more intense.
I think introducing Infected as playable characters in MP would be very tricky.
If you wanna check something similar, try Dead Space 2's MP. It is actually quite good.
I think balancing a 'human' vs infected style of game would be hard, specially if it is a deathmatch kind of game.
I have suggested how Infected could be playable in Free-4-All modes. Basically, if you die during a certain time, in a certain place, or in a certain way; you spawn as an Infected in your next life. Kinda like a U3 powerplay, though the spawining as an Infected could actually count against you, as it would halt your scoring in regards to the objetives.
In team modes, an (powerplay) Infected player could attempt to cheat by not attacking his 'human' teammates. So, it this could be bad. Seeing all 'humans' as equal, not identifying their team, could help; like U3 curse.
I think Infected in the MP should have some sort of sense of smell, which could work like a radar, so they could easily find their prey.
I like the idea of pressing a button to make a clicking sound as a clicker, and the other infected can see (Except for when they die or are clickers and past) And I think that humans should see Infected players as a threat, like you would in the main campaign. The Infected would have devastating amounts of damage delivery against you as a human, making you try your best to avoid them and take them on using stratedgy. Plus the mode of one player hunting as a clicker turning humans into runners until the time limit runs out or all of the humans have turned. Now, in deathmatch modes humans would have the advantage of guns and weapons, but all the runners can do is melee, so they have to be powerful.