From the Valve Developer Wiki...
https://developer.valvesoftware.com/wiki/Left_4_Dead_Infected_Populations

Infected populations control which kinds of common infected can be spawned in the different sections of your map.

For example, infected populations can cause:

• Patients, nurses, and surgeons to spawn in a hospital
  
• Military personnel to spawn in a military base
  
• Rural citizens to spawn on a farm
  
• Police officers to spawn on a street filled with police cars

C1M1 Hotel
Default Population = "c1streets"
Additional Population = "default" (Used by the intro animation for zombies to die in the fire)


C1M2 Streets
Default Population = "c1streets"


C1M3 Mall
Default Population = "default"


C1M4 Atrium
Default Population = "default"
Additional Population = "default" (Used by the outro animation for zombies to die in the car collision)

C2M1 Highway
Default Population = "default"


C2M2 Fairgrounds
Default Population = "whisperingoaks"


C2M3 Coaster
Default Population = "whisperingoaks"


C2M4 Barns
Default Population = "whisperingoaks"


C2M5 Concert
Default Population = "whisperingoaks"

C3M1 Plank Country
Default Population = "gatorpark"


C3M2 Swamp
Default Population = "deepswamp"
Additional Population = "deepswamp_parachutist" (Added in The Last Stand Update, this covers most of the map, notably around the dead parachutist and the crashed airplane.)


C3M3 Shanty Town
Default Population = "deepswamp"


C3M4 Plantation
Default Population = "default"

C4M1 Milltown
Default Population = "defaultmilltown"


C4M2 Sugar Mill
Default Population = "defaultsugarmill"


C4M3 Mill Escape
Default Population = "defaultsugarmill"


C4M4 Return to Town
Default Population = "defaultmilltown"


C4M5 Town Escape
Default Population = "defaultmilltown"

C5M1 Waterfront
Default Population = "crescentcity"


C5M2 Park
Default Population = "crescentcity"


C5M3 Cemetary
Default Population = "defaultcemetary"


C5M4 Quarter
Default Population = "frenchquarter"
Additional Population = "ceda" (Only near the end saferoom)


C5M5 Bridge
Default Population = "default"

C6M1 Riverbank
Default Population = "riverbank"
Additional Population = "riverbank_wedding" (At the park and wedding area)


C6M2 Underground
Default Population = "redlight"
Additional Population = "redlight_bikers" (Inside the bars and restaurants)


C6M3 Port
Default Population = "port"

C7M1 The Docks
Default Population = "river"
Additional Population = "riverintro" (Used by the intro animation for zombies that get killed)
Additional Population = "riverarena" (The tank fight arena)
Additional Population = "riverfreighter" (The brick factory)


C7M2 The Barge
Default Population = "river"
Additional Population = "riverfreighter" (The barge panic event)
Additional Population = "riverarena" (Outside the end saferoom)


C7M3 The Port
Default Population = "riverport"
Additional Population = "riverport" (Almost the entire nav is marked as 'riverport' even though it doesn't need to be because the default is already 'riverport'.)

C8M1 The Apartments
Default Population = "defaulturban_l4d1"


C8M2 The Subway
Default Population = "defaulturban_l4d1"


C8M3 The Sewer
Default Population = "maintenancestreet_l4d1"
Additional Population = "maintenancesewer_l4d1" (Inside the sewers)
Additional Population = "hospitalgrounds_l4d1" (The area outside the hospital)


C8M4 The Hospital
Default Population = "hospital_l4d1"
Additional Population = "hospital_l4d1" (Yeah, it's redundant)
Additional Population = "hospitalconstruction_l4d1" (The upper floor, past the elevator)


C8M5 The Rooftop
Default Population = "hospitalrooftop_l4d1"

C9M1 The Alleys
Default Population = "default_l4d1"


C9M2 The Truck Depot
Default Population = "defaultlots_l4d1"

C10M1 The Turnpike
Default Population = "default_l4d1"


C10M2 The Drains
Default Population = "defaultrural_l4d1"
Additional Population = "ruralsewer_l4d1" (Most of the map uses this, only the area outside the end saferoom doesn't)


C10M3 The Church
Default Population = "defaultrural_l4d1"


C10M4 The Town
Default Population = "ruralstreet_l4d1"


C10M5 The Boathouse
Default Population = "defaultrural_l4d1"

C11M1 The Greenhouse
Default Population = "defaulturban_l4d1"


C11M2 The Crane
Default Population = "defaulturban_l4d1"


C11M3 The Construction Site
Default Population = "defaulturban_l4d1"
Additional Population = "police_l4d1" (The police car in the street after the panic event is the only location in L4D1 or L4D2 that uses this population. Neat!)
Additional Population = "maintenance_l4d1" (The electrical substation area)
Additional Population = "constructionzone_l4d1" (The panic event area)


C11M4 The Terminal
Default Population = "airport_l4d1"
Additional Population = "airport_l4d1" (Used by info_zombie spawners during the metal detector panic event. Very redundant. Why'd you do this, Valve?)


C11M5 The Runway
Default Population = "airportfinale_l4d1"

C12M1 The Woods
Default Population = "defaultrural_l4d1"


C12M2 The Tunnel
Default Population = "defaultrural_l4d1"


C12M3 The Bridge
Default Population = "defaultrural_l4d1"


C12M4 The Train Station
Default Population = "defaultrural_l4d1"


C12M5 The Farmhouse
Default Population = "defaultrural_l4d1"

C13M1 Alpine Creek
Default Population = "default"


C13M2 South Pine Stream
Default Population = "default"
Additional Population = "deepswamp" (The river and panic event tunnel)


C13M3 Memorial Bridge
Default Population = "default"
Additional Population = "busstation" (Most of the map uses this)


C13M4 Cut-throat Creek
Default Population = "default"

C14M1 The Junkyard
Default Population = "junkyardrural_l4d1"
Additional Population = "junkyardroad_l4d1" (The starting area)
Additional Population = "estuary_l4d1" (The water near the bridge)
Additional Population = "junkyard_l4d1" (The junkyard panic event area)


C14M2 The Lighthouse
Default Population = "lighthouserural_l4d1"
Additional Population = "lighthouse_l4d1" (The finale area around the lighthouse)

Swamp Fever
"defaultswamp"

Hard Rain
"defaultmilltown_mob"
"defaultsugarmill_mob"
"defaultmilltown_rain"
"defaultmilltown_rain_mob"
"defaultsugarmill_rain"
"defaultsugarmill_rain_mob"

The Passing
"riverbank_police"

The Sacrifice
"riveratrium"

Unused L4D1 Populations added in 'The Last Stand Update'
"riotpolice_l4d1"
"military_l4d1"

The Sacrifice Unused L4D1 Populations
"river_l4d1"
"riverintro_l4d1"
"riverarena_l4d1"
"riveratrium_l4d1"
"riverfreighter_l4d1"
"riverport_l4d1"

No longer used officially after 'The Last Stand Update'
"defaulturban"
"police"
"maintenance"
"hospitalgrounds"
"hospital"
"hospitalconstruction"
"airport"
"airportfinale"
"defaultrural"