Valve wrote on their official Left 4 Dead blog that the developer was "adding bots for the Infected team to help out the Infected when they are down a man or two." This means anyone will appreciate this change if they love being a Hunter or a Boomer. This is great news indeed for L4D fans as the auto spawn has left the Infected team with no option to retaliate (and time being an enemy); however the addition of Infected bots will give fans a chance to defeat the lowly survivors without waiting.
The blog post also states that Valve is already hard at work on a new DLC campaign called "The Passing." Valve has given no specific release date but we can expect it sometime before summer.