The Hospital

The Hospital is the fourth chapter of the first campaign, No Mercy. It takes place in the city of Fairfield, inside of the overwhelmed Mercy Hospital. The goal of the Survivors here is to get to the roof, where they will be able to make contact with the helicopter which has been flying around the city, while fighting hordes of Infected doctors, patients, and other people.

Campaign
At the beginning, the AI Survivors will grab:
 * Bill will grab an assault rifle.
 * Francis will grab an auto shotgun.
 * Louis will grab an assault rifle.
 * Zoey will grab a hunting rifle.

Otherwise, they will keep the weapons they had in the last chapter (see The Sewer for a list of weapons they will grab there).

It's been a long run. Through apartments, abandoned subways, and sewers you've gone. But you're here now. Rescue awaits you on the roof of the hospital. Now, all you have to do is get everyone there alive, and you'll have made it. Take a breather, if you must, and be prepared to run through the forsaken halls of Mercy Hospital.

Of course, you start this level out in a safe room. There are tier 2 weapons sitting on the table, as well as an extra pistol and a box first aid kits on the wall. Stock up and get ready to move. Due to the closed quarters of this level, an auto shotgun is recommended. Take whatever you'd like, open that door, and head out.

Head to the right of the door, down the hall, and head for the light at the end which leads to some stairs. You can go to the left, though, and look for supplies, or turn at the bend into a darkened hallway, as well. However, both are ultimately dead ends. Go up the flight of stairs and onto the next floor. Take a look around, if you must, but the way to keep going is to the left; either by the immediate route, or a small detour through a few rooms.
 *  This is one of the most likely times for a Tank or Witch to appear (especially in versus mode). If you hear either of them, get ready to either fight or try to file around the Witch (if there is one), although they usually spawn directly in the way. 

Head for the stairs on the far side of the room, possibly pausing to look inside of the few rooms along the way if you really want supplies, and go down the hall until you reach more stairs. Go up, and keep moving.
 *  This is another very likely spot for a Tank or Witch to appear. From this point to the elevator, watch out! 

Take a look behind the desk here. There tend to be some supplies around this area, if the Director is forgiving. Move down a rather long hallway, lined with beds and gurneys, and you'll find yourself at an elevator. Don't you just hate waiting for the elevator when you've somewhere to be? Don't hit the button yet, though! There's a pile of ammo sitting on a bed blocking off a corner (which is a very common tactic to use for this section, though it's mostly when there are other players), and sometimes supplies are hiding off in one of the adjacent rooms down this hall. Take a look around and make use of what you can, and only when you and everyone else is ready, hit the button and get ready to wait. Oh, and there's a horde coming.

They'll be coming in from almost every direction—even dropping down from a vent over your head, breaking down walls all around you. At this point, though, there are a few different tactics which can be used to great effect:
 * Pipe bombs, obviously. Throw as many as you've got, and it easily thins out the horde.
 * Molotovs. Throw them down at one of the pathways the Infected will be charging in from, and it does most of the work for you.
 * There is a small closet on the other side of the wall where the elevator and bed are. Everyone can go in there, but be careful of special Infected that drop out of the ceiling!
 * As aforementioned, everyone can get in the corner behind the bed. Not only does the bed keep anybody from being dragged away by a Smoker, but the wall also covers your backs quite well.

In general, just stay together, preferably close to the elevator, and you should be fine. Once the elevator finally gets to your floor, hurry and get on before more come running.

As the elevator slowly goes up, this would be a nice time to work out who needs health. Figure it out, but be ready to hit the ground running when it reaches its destination—the 28th floor.
 *  This is the final place a Tank or Witch generally spawn, usually farther down and not far from the safe room. Needless to say, with all of the holes in the walls, the Tank is definitely the more dangerous of the two here. Just avoid the open points in the walls and shoot as much as you can. 

You're almost there. The elevator lets you out to an unfinished part of the hospital, which is quite obviously still under heavy construction, judging by the wooden beams everywhere... as well as the lack of outer walls. You're almost to the safe house, though! Run through the area (carefully, you don't want to fall off the roof!), and you'll have made it. Be careful, if a tank spawns here he can send you flying off the roof. Get everyone inside, and shut the door when they are. Congratulations, you've made it through The Hospital! Take a moment to catch your breath, and get ready for the Rooftop Finale.

Bugs
All of these bugs have been fixed on both the PC and Xbox 360 version.
 * After the battle with the horde at the elevator, if a Survivor managed to become incapacitated inside the elevator for any reason and was revived once the elevator had been activated, the elevator rose but they did not. They would also have been unable to leave the elevator shaft, and have been easy prey to any other Infected.
 * Throwing a propane tank or oxygen tank at a Survivor in the lift would incapacitate them and cause them to fall through the floor of the lift, leaving them at the bottom of the elevator shaft.
 * As a Smoker, it was possible to pull a Survivor out of the elevator just as the doors were closing.
 * In Versus mode, any of the Special Infected (except the Tank) were able to run ahead in the level to the top of the elevator shaft and repeatedly melee the trailer in front of the elevator, eventually pushing it in front of the doors and trapping the Survivors when they reached the top. The Tank can still hit the trailer and move it in front of the elevator, but objects hit by the Tank eventually disappear, making this a temporary obstruction.

Trivia

 * This level is often considered one of the most liked levels in Left 4 Dead. Also one of the scariest, since patient and doctor zombies tend to appear from dark and silent rooms, and hospitals are always the scariest places in zombie themed movies/games.
 * In several of the examining rooms, there is an x-ray of a hand with four fingers, which is a reference to the cover of the game.
 * In the chapter's starting safe room, there is some Graffiti on the wall, one area of which will say that "I killed..." some number of zombies, the last number of which 53,595. This is a reference to the Zombie Genocide achievement from the popular zombie hack and slash video game Dead Rising, and that game's setting of the town of Willamette, Colorado, where the population is 53,594.
 * As you start the game, run up the stairs and into the large main room on that floor, to the right of the large door you most likely entered by, there is a door handle on the wall (more than likely an oversight).
 * An effective yet 'cheap' as it's considered by most players strategy; While you are on the infected team in versus, make sure your team is all hunters.(Note: This takes quite an amount of teamwork) Make your way to the elevator shaft before the Survivor team does, and tell all 4 of your team members to get in the elevator before they press the button to call the elevator to get them. Once they do, get your team to prepare themselves and as soon as the door of the elevator opens, the survivors will probably not be aware of your presence and will try to walk in as if your team is not there. Before they have a chance to do so, your team should each pounce on one survivor, this will kill all four of them quickly and easily.
 * The Hospital level may be a reference or homage to the 2002 film 28 Days Later where the main character wakes up from a coma to find the hospital completely deserted.