Left 4 Dead 2 introduced seven special types of Common Infected known as "Uncommon Infected". Each Uncommon Infected is unique to the campaign it appears in (excluding DLC campaigns) and has its own special ability that makes it slightly to moderately more dangerous than Common Infected.
Dead Center features the CEDA Worker Infected (Immune to flames, improved health, low chance to drop a Bile Bomb when defeated) and Jimmy Gibbs Junior (Immune to flames, as tough as a Witch, but can be killed with a headshot instantly).
The Passing features the Fallen Survivor (May drop first aid kit, Molotov, pipe bomb or pain pills and has increased health. Their helmet makes it harder to score a headshot, and run away when damaged).
Dark Carnival features the Clown Infected (which have squeaky shoes that attract nearby Common Infected).
Swamp Fever features the Mud Men (each hit slightly blinds a Survivor's vision, and they also crawl swiftly around on all fours, not to mention they may become hard to spot, but Pipe Bombs can still easily kill a large amount of them).
Hard Rain features the Worker Infected (who have earmuffs, so they don't chase Pipe bombs, and only chase Bile Bombs if it hits any infected. The hard hats seem to be only cosmetic, as they don't prevent them from being headshot though they may simply not be rated for ballistic protection).
The Parish features the Riot Infected (Who can only be killed by attacking them from behind due to their body armor, but it's main weakness is a Chainsaw that instantly kills it, also has a small chance to drop Nightstick).
Uncommon Infected can also be found in DLC campaigns for Left 4 Dead 2.
No Mercy and The Sacrifice feature both CEDA Worker Infected and Worker Infected.
Cold Stream features both Mud Men and Riot Infected.
(All done!)