How can I avoid being detected as an infected in left 4 dead 2?

Don't spawn until the last possible moment or they'll hear you.

Here are some of my strategies:

• Boomer - He's slow, so don't try chasing survivors. Get in front of them and spawn as they're coming around a corner, or drop from a higher place in between them and surprise 'em.
• Smoker - I usually wait for a Boomer to attack and as soon as he explodes on them I grab one, because they're all blind and deaf for a few seconds. This gives me enough time to do some damage. He's also good at grabbing survivors off ledges and high places.
• Charger - I wait for the survivors to get in a cluster like in a narrow passage and then try to ram them all as it will take a couple of seconds for them to get up and kill you. He's also good to lead survivors back and to delay them by charging one of them across a long distance.
• Spitter - Just hide and spit. Keep out of sight, as she is slow and dies quickly. Your spit can bounce off walls; use that to spit without exposing yourself. If they've been hit by a Boomer and are getting attacked by common infected then spit at them, as attacking horde slows them down.
• Jockey - Not a big fan, but I normally wait until the others attack and then sneak in, grab one, and ride off into the sunset. You can use ledges and cliffs to your advantage and walk a survivor off them. Even if they hang on, the rest of the team will either need to go back and lift him up, or leave him hanging and play the rest of the level down one survivor. Also, works well in tandem with a Spitter.
• Hunter - Hunter is silent when standing and also runs fast; use that to run up to weak survivors from behind and do a scratch and run. Also, pouncing from very high will do a lot of damage. I normally use him to punish any survivors trying to free a teammate from a Smoker or Charger.

Hang back and never attack a group. Wait for one of the survivors to lag behind and then act quickly.

Most of the time the survivors won't bother doubling back to kill you, so staying behind them will help you stay alive longer.

Also: teamwork! Work with your teammates and coordinate your attacks. Again, act swiftly.

Don't hide and kill as fast as possible. They won't hear you if they are already dead. :)