Left 4 Dead 1 and 2 New add on?

[Doctor Nitrus Brio]

I was watching a video on X-box about Left 4 Dead 2 having an add on. Since I was heavily distracted while trying to watch it, can someone explain it to me?

I know that the cast from part 1 are going to be in it and some zombies will have med-kits on them... I believe a new game mode will come too.

Left 4 Dead 1 will explain why they, cast from l4d 1, are in l4d 2 in a new add-on.  

[The Goat King]

The Have Confirmed That In The Add On Mission, The Passing (In Which They Meet Up With The Louis,Bill,Francis, And Zoey,Doing Something That Has Not Been Anounced.)
There Is A Suriviors/Psychopaths/Zombies With Weapons But Will Mainly Run A Way,And You Can Kill Them For Supply,And Rumored That They Will Have Add Ons For More Zombies,Such As The Cut Out Screamer,And More.  

[Tomha_Domizoko]

Here's a Link with Details.

http://left4dead.wikia.com/wiki/The_Passing  

[Garret Erikson]

The zombies with the healthkits are Fallen Survivors. They are like CEDA zombies, in the way that some will drop items when killed, but it won't always be the same thing.

And I'm sorry ifin it disenchants you Repo, but the Screamer will probably never be in the game. There were rumors about it back when Survival Mode came out, and then again with Crash Course. People on the internet just like making s**t up.  

[Doctor Nitrus Brio]

Thanks GUys