Do you have an old neighbor who sits on his porch and yells racial slurs all day? Of course you do, and when he was in the war he ran around with a rocket launcher and cleared trenches.
Think of this map like non-linear fatkid except the fatkid has rockets and the infected have rifles.
Young Grandpa- He has a rocket launcher, is invincible, and is fairly slow. If you see him, you are probably dead.
Soldier (US Soldier)-Your goal is to survive, find good weapons, and defend yourself from the regular infected while avoiding the onslaught, there are plenty of weapons and even a few holdouts on the map to temporarily control until you get destroyed.
Regular infected (**** Troops)- You are extremely weak and your rifle takes little damage, however in groups you are incredibly overwhelming, while you might take little damage you can be sneaky and assassinate the soldiers.