<s>
Left	B-Application
4	I-Application
Dead	I-Application
is	O
a	O
2008	O
first-person	O
shooter	O
game	O
developed	O
by	O
Valve	O
South	O
and	O
published	O
by	O
Valve	O
.	O
</s>
<s>
It	O
was	O
originally	O
released	O
for	O
Microsoft	B-Application
Windows	I-Application
and	O
Xbox	B-Operating_System
360	I-Operating_System
in	O
November	O
2008	O
and	O
for	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
in	O
October	O
2010	O
,	O
and	O
is	O
the	O
first	O
title	O
in	O
the	O
Left	B-Application
4	I-Application
Dead	I-Application
series	O
.	O
</s>
<s>
Set	O
during	O
the	O
aftermath	O
of	O
a	O
zombie	B-Application
outbreak	O
on	O
the	O
East	O
Coast	O
of	O
the	O
United	O
States	O
,	O
the	O
game	O
pits	O
its	O
four	O
protagonists	O
,	O
dubbed	O
the	O
"	O
Survivors	O
"	O
,	O
against	O
hordes	O
of	O
the	O
infected	O
.	O
</s>
<s>
Left	B-Application
4	I-Application
Dead	I-Application
uses	O
Valve	O
's	O
proprietary	O
Source	B-Application
engine	I-Application
,	O
with	O
four	O
game	O
modes	O
:	O
a	O
single-player	O
mode	O
in	O
which	O
allied	O
characters	O
are	O
controlled	O
by	O
AI	B-Application
,	O
a	O
four-player	O
co-op	O
campaign	O
mode	O
,	O
an	O
eight-player	O
online	O
versus	O
mode	O
,	O
and	O
a	O
four-player	O
survival	O
mode	O
.	O
</s>
<s>
In	O
all	O
modes	O
,	O
an	O
artificial	B-Application
intelligence	I-Application
dubbed	O
the	O
"	O
Director	O
"	O
controls	O
level	O
pacing	O
and	O
item	O
placements	O
in	O
an	O
attempt	O
to	O
create	O
a	O
dynamic	O
experience	O
and	O
increase	O
replay	O
value	O
.	O
</s>
<s>
Left	B-Application
4	I-Application
Dead	I-Application
was	O
well-received	O
by	O
the	O
industry	O
media	O
upon	O
its	O
release	O
,	O
with	O
praise	O
for	O
its	O
replay	O
value	O
,	O
focus	O
on	O
cooperative	O
play	O
,	O
and	O
cinematic	O
feel	O
,	O
although	O
some	O
criticized	O
its	O
limited	O
level	O
selection	O
and	O
lack	O
of	O
narrative	O
.	O
</s>
<s>
As	O
was	O
done	O
with	O
Team	B-Application
Fortress	I-Application
2	I-Application
,	O
Valve	O
supplemented	O
the	O
game	O
with	O
free	O
downloadable	B-Protocol
content	I-Protocol
.	O
</s>
<s>
The	O
success	O
of	O
the	O
game	O
led	O
to	O
the	O
development	O
of	O
the	O
sequel	B-Application
Left	B-Application
4	I-Application
Dead	I-Application
2	I-Application
,	O
which	O
was	O
released	O
in	O
2009	O
.	O
</s>
<s>
In	O
2012	O
,	O
all	O
Left	B-Application
4	I-Application
Dead	I-Application
campaigns	O
were	O
ported	O
over	O
to	O
Left	B-Application
4	I-Application
Dead	I-Application
2	I-Application
,	O
with	O
cross-platform	O
multiplayer	O
support	O
between	O
Windows	O
and	O
Mac	O
versions	O
of	O
the	O
game	O
.	O
</s>
<s>
An	O
outbreak	O
of	O
a	O
highly	O
contagious	O
pathogen	O
nicknamed	O
the	O
"	O
Green	O
Flu	O
"	O
(	O
which	O
causes	O
extreme	O
aggression	O
,	O
mutation	O
to	O
the	O
body	O
cells	O
,	O
loss	O
of	O
higher	O
brain	O
functions	O
,	O
and	O
essential	O
zombification	B-Application
of	O
those	O
who	O
catch	O
it	O
)	O
begins	O
in	O
Pennsylvania	O
.	O
</s>
<s>
After	O
narrowly	O
avoiding	O
the	O
new	O
types	O
of	O
infected	O
,	O
the	O
survivors	O
are	O
alerted	O
by	O
a	O
passing	O
helicopter	B-Application
to	O
an	O
evacuation	O
point	O
at	O
the	O
roof	O
of	O
Mercy	O
Hospital	O
.	O
</s>
<s>
Zoey	O
is	O
forced	O
to	O
kill	O
him	O
,	O
causing	O
the	O
helicopter	B-Application
to	O
crash	O
in	O
an	O
industrial	O
district	O
outside	O
the	O
city	O
.	O
</s>
<s>
Like	O
the	O
helicopter	B-Application
before	O
it	O
,	O
this	O
plane	O
also	O
crashes	O
,	O
and	O
the	O
survivors	O
find	O
themselves	O
at	O
the	O
outskirts	O
of	O
the	O
Allegheny	O
National	O
Forest	O
.	O
</s>
<s>
While	O
waiting	O
for	O
the	O
horde	O
to	O
disperse	O
,	O
the	O
three	O
remaining	O
members	O
encounter	O
the	O
survivors	O
from	O
Left	B-Application
4	I-Application
Dead	I-Application
2	I-Application
.	O
</s>
<s>
Left	B-Application
4	I-Application
Dead	I-Application
is	O
a	O
first-person	O
shooter	O
in	O
which	O
the	O
player	O
takes	O
control	O
of	O
one	O
of	O
four	O
survivors	O
;	O
if	O
human	O
players	O
are	O
not	O
available	O
,	O
the	O
remaining	O
survivors	O
are	O
filled	O
by	O
AI-controlled	O
bots	O
.	O
</s>
<s>
They	O
play	O
through	O
campaigns	O
fighting	O
off	O
the	O
infected	O
—	O
living	O
humans	O
who	O
have	O
been	O
infected	O
with	O
a	O
rabies-like	O
virus	O
that	O
causes	O
psychosis	B-Application
.	O
</s>
<s>
This	O
is	O
compounded	O
by	O
the	O
presence	O
of	O
an	O
"	O
AI	B-Application
Director	O
"	O
,	O
who	O
tracks	O
each	O
player	O
's	O
current	O
situation	O
,	O
and	O
will	O
add	O
or	O
remove	O
items	O
and	O
infected	O
in	O
response	O
;	O
with	O
the	O
goal	O
of	O
maintaining	O
constant	O
tension	O
and	O
creating	O
a	O
new	O
experience	O
with	O
each	O
playthrough	O
.	O
</s>
<s>
The	O
AI	B-Application
will	O
not	O
open	O
"	O
respawn	O
closets	O
"	O
,	O
so	O
if	O
all	O
human	O
player	O
survivors	O
are	O
incapacitated	O
,	O
the	O
level	O
will	O
restart	O
.	O
</s>
<s>
Left	B-Application
4	I-Application
Dead	I-Application
has	O
friendly	O
fire	O
,	O
increasing	O
the	O
need	O
for	O
caution	O
and	O
coordination	O
around	O
other	O
survivors	O
.	O
</s>
<s>
Additional	O
communication	O
of	O
player	O
actions	O
is	O
conveyed	O
through	O
lights	O
;	O
weapon-mounted	O
flashlights	O
and	O
muzzle	O
flashes	O
help	O
players	O
determine	O
when	O
their	O
companions	O
are	O
shooting	O
,	O
performing	O
melee	B-Application
attacks	O
,	O
reloading	O
or	O
moving	O
.	O
</s>
<s>
(	O
Due	O
to	O
control	O
issues	O
and	O
the	O
likelihood	O
of	O
players	O
using	O
a	O
headset	O
,	O
the	B-Operating_System
Xbox	I-Operating_System
360	I-Operating_System
version	O
of	O
Left	B-Application
4	I-Application
Dead	I-Application
omits	O
the	O
quick	O
phrases	O
feature	O
.	O
)	O
</s>
<s>
A	O
two-level	O
DLC	B-Protocol
campaign	O
,	O
"	O
Crash	O
Course	O
"	O
,	O
was	O
released	O
on	O
September	O
29	O
,	O
2009	O
,	O
set	O
in	O
the	O
industrial	O
outskirts	O
between	O
"	O
No	O
Mercy	O
"	O
and	O
"	O
Death	O
Toll	O
"	O
.	O
</s>
<s>
A	O
melee	B-Application
attack	O
can	O
always	O
be	O
used	O
to	O
shove	O
away	O
any	O
infected	O
within	O
reach	O
.	O
</s>
<s>
The	O
infected	O
(	O
voiced	O
by	O
experimental	O
musician	O
Mike	O
Patton	O
[	O
commons	O
,	O
Smoker	O
,	O
and	O
Hunter ]	O
and	O
voice	O
actor	O
Fred	O
Tatasciore	O
[	O
Boomer	O
and	O
Tank ]	O
)	O
are	O
the	O
enemies	O
of	O
Left	B-Application
4	I-Application
Dead	I-Application
,	O
and	O
comparisons	O
have	O
been	O
made	O
with	O
'	O
zombies	B-Application
 '	O
from	O
certain	O
modern	O
films	O
,	O
such	O
as	O
Dawn	O
of	O
the	O
Dead	O
,	O
28	O
Days	O
Later	O
,	O
and	O
Quarantine	O
.	O
</s>
<s>
An	O
important	O
distinction	O
from	O
the	O
former	O
is	O
that	O
the	O
infected	O
are	O
,	O
as	O
cited	O
in	O
the	O
game	O
's	O
manual	O
,	O
living	O
humans	O
infected	O
with	O
a	O
rabies-like	O
pathogen	O
;	O
more	O
similar	O
to	O
the	O
latter	O
two	O
films	O
,	O
rather	O
than	O
undead	O
zombies	B-Application
.	O
</s>
<s>
There	O
are	O
five	O
special	O
infected	O
in	O
Left	B-Application
4	I-Application
Dead	I-Application
:	O
</s>
<s>
The	O
Witch	B-Application
is	O
a	O
female	O
infected	O
with	O
long	O
claws	O
,	O
who	O
stays	O
still	O
in	O
one	O
spot	O
.	O
</s>
<s>
Survivors	O
can	O
often	O
avoid	O
the	O
Witch	B-Application
,	O
but	O
if	O
they	O
disturb	O
her	O
with	O
light	O
,	O
loud	O
noises	O
,	O
gunfire	O
,	O
or	O
getting	O
too	O
close	O
,	O
she	O
will	O
become	O
aggressive	O
and	O
charge	O
.	O
</s>
<s>
The	O
Witch	B-Application
is	O
the	O
strongest	O
infected	O
in	O
the	O
game	O
,	O
and	O
on	O
the	O
hardest	O
difficulties	O
,	O
will	O
kill	O
a	O
survivor	O
with	O
one	O
hit	O
.	O
</s>
<s>
The	O
artificial	B-Application
intelligence	I-Application
of	O
Left	B-Application
4	I-Application
Dead	I-Application
features	O
a	O
dynamic	O
system	O
for	O
game	O
dramatics	O
,	O
pacing	O
and	O
difficulty	O
called	O
the	O
"	O
Director	O
"	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
AI	B-Application
Director	O
,	O
there	O
is	O
a	O
second	O
Director	O
that	O
controls	O
music	O
.	O
</s>
<s>
Director	O
was	O
previously	O
used	O
for	O
some	O
key	O
battles	O
in	O
Half-Life	B-Application
2	I-Application
:	I-Application
Episode	I-Application
Two	I-Application
.	O
</s>
<s>
Left	B-Application
4	I-Application
Dead	I-Application
contains	O
four	O
game	O
modes	O
:	O
Campaign	O
,	O
Versus	O
,	O
Survival	O
,	O
and	O
Single-Player	O
.	O
</s>
<s>
In	O
this	O
mode	O
,	O
the	O
special	O
infected	O
are	O
controlled	O
by	O
the	O
AI	B-Application
.	O
</s>
<s>
Survival	O
mode	O
consists	O
of	O
a	O
timed	O
challenge	O
where	O
players	O
try	O
to	O
survive	O
as	O
long	O
as	O
possible	O
against	O
a	O
never-ending	O
flood	O
of	O
the	O
infected	O
,	O
added	O
in	O
April	O
2009	O
in	O
the	O
Survival	O
Pack	O
DLC	B-Protocol
.	O
</s>
<s>
Single-player	O
mode	O
is	O
similar	O
to	O
campaign	O
mode	O
,	O
but	O
played	O
offline	O
with	O
three	O
AI-controlled	O
bots	O
as	O
the	O
other	O
survivors	O
.	O
</s>
<s>
On	O
Xbox	B-Operating_System
360	I-Operating_System
,	O
other	O
players	O
can	O
join	O
the	O
same	O
console	O
to	O
turn	O
single	O
player	O
into	O
an	O
offline	O
co-op	O
game	O
.	O
</s>
<s>
Left	B-Application
4	I-Application
Dead	I-Application
can	O
also	O
be	O
played	O
through	O
a	O
system	B-Device
link	I-Device
,	O
or	O
local	O
area	O
network	O
.	O
</s>
<s>
Players	O
have	O
also	O
discovered	O
a	O
way	O
to	O
do	O
splitscreen	O
co-op	O
with	O
the	O
PC	B-Device
version	O
.	O
</s>
<s>
Development	O
on	O
Left	B-Application
4	I-Application
Dead	I-Application
started	O
in	O
mid-2005	O
.	O
</s>
<s>
The	O
game	O
was	O
first	O
revealed	O
in	O
the	O
Christmas	O
2006	O
publication	O
of	O
PC	B-Device
Gamer	O
UK	O
with	O
a	O
six-page	O
article	O
describing	O
a	O
playthrough	O
at	O
Turtle	O
Rock	O
Studios	O
headquarters	O
.	O
</s>
<s>
A	O
teaser	O
was	O
released	O
with	O
The	B-Application
Orange	I-Application
Box	I-Application
.	O
</s>
<s>
Turtle	O
Rock	O
Studios	O
announced	O
Left	B-Application
4	I-Application
Dead	I-Application
on	O
November	O
20	O
,	O
2006	O
,	O
and	O
was	O
acquired	O
by	O
Valve	O
on	O
January	O
10	O
,	O
2008	O
,	O
because	O
of	O
the	O
game	O
and	O
long-standing	O
relationship	O
between	O
the	O
companies	O
.	O
</s>
<s>
The	O
game	O
opened	O
up	O
to	O
pre-purchasing	O
on	O
Valve	O
's	O
Steam	B-Application
system	I-Application
on	O
October	O
15	O
,	O
2008	O
.	O
</s>
<s>
To	O
give	O
Left	B-Application
4	I-Application
Dead	I-Application
significant	O
exposure	O
,	O
Valve	O
financed	O
a	O
$	O
10-million	O
marketing	O
campaign	O
for	O
the	O
game	O
in	O
the	O
United	O
States	O
and	O
Europe	O
,	O
with	O
advertisements	O
appearing	O
on	O
television	O
,	O
print	O
,	O
websites	O
and	O
outdoor	O
placements	O
in	O
many	O
cities	O
.	O
</s>
<s>
offering	O
copies	O
of	O
Left	B-Application
4	I-Application
Dead	I-Application
to	O
people	O
who	O
submitted	O
the	O
best	O
picture	O
involving	O
zombies	B-Application
or	O
the	O
outdoor	O
advertising	O
.	O
</s>
<s>
Left	B-Application
4	I-Application
Dead	I-Application
uses	O
the	O
2008	O
version	O
of	O
Valve	O
's	O
Source	B-Application
engine	I-Application
,	O
with	O
improvements	O
such	O
as	O
multi-core	B-Architecture
processor	I-Architecture
support	O
and	O
physics-based	O
animation	O
to	O
more	O
realistically	O
portray	O
hair	O
and	O
clothing	O
,	O
and	O
to	O
improve	O
physics	O
interaction	O
with	O
enemies	O
when	O
shot	O
or	O
shoved	O
in	O
different	O
body	O
parts	O
.	O
</s>
<s>
Rendering	O
and	O
artificial	B-Application
intelligence	I-Application
were	O
scaled	O
up	O
to	O
allow	O
for	O
a	O
greater	O
number	O
of	O
enemies	O
who	O
can	O
navigate	O
the	O
world	O
in	O
better	O
ways	O
,	O
such	O
as	O
climbing	O
,	O
jumping	O
,	O
or	O
breaking	O
obstacles	O
.	O
</s>
<s>
Lighting	O
was	O
enhanced	O
with	O
new	O
self-shadowing	O
normal	B-Application
mapping	I-Application
and	O
advanced	O
shadow	O
rendering	O
that	O
is	O
important	O
to	O
convey	O
information	O
about	O
the	O
environment	O
and	O
player	O
actions	O
.	O
</s>
<s>
There	O
is	O
dynamic	O
color	O
correction	O
that	O
accentuates	O
details	O
based	O
on	O
importance	O
,	O
contrast	O
and	O
sharpening	B-Algorithm
to	O
focus	O
attention	O
on	O
critical	O
areas	O
,	O
film	O
grain	O
to	O
expose	O
details	O
or	O
imply	O
details	O
in	O
dark	O
areas	O
and	O
vignetting	O
to	O
evoke	O
tension	O
and	O
a	O
horror-film	O
look	O
.	O
</s>
<s>
Left	B-Application
4	I-Application
Dead	I-Application
underwent	O
many	O
phases	O
of	O
development	O
;	O
influenced	O
by	O
playtesting	O
,	O
Turtle	O
Rock	O
Studios	O
removed	O
many	O
of	O
the	O
features	O
that	O
were	O
originally	O
in	O
the	O
game	O
.	O
</s>
<s>
Certain	O
Affinity	O
assisted	O
Turtle	O
Rock	O
Studios	O
with	O
the	B-Operating_System
Xbox	I-Operating_System
360	I-Operating_System
version	O
of	O
the	O
game	O
.	O
</s>
<s>
The	B-Operating_System
Xbox	I-Operating_System
360	I-Operating_System
version	O
of	O
Left	B-Application
4	I-Application
Dead	I-Application
has	O
the	O
same	O
game	O
modes	O
as	O
the	O
PC	B-Device
version	O
but	O
has	O
additional	O
features	O
such	O
as	O
support	O
for	O
split	O
screen	O
,	O
allowing	O
for	O
two	O
players	O
to	O
play	O
offline	O
and	O
online	O
from	O
the	O
same	O
console	O
,	O
and	O
System	B-Device
Link	I-Device
play	O
.	O
</s>
<s>
Split-screen	O
mode	O
can	O
also	O
be	O
achieved	O
on	O
the	O
PC	B-Device
version	O
,	O
but	O
it	O
requires	O
console	O
commands	O
and	O
may	O
require	O
the	O
modification	O
of	O
controller	O
configuration	O
files	O
;	O
and	O
it	O
is	O
not	O
officially	O
supported	O
.	O
</s>
<s>
This	O
new	O
server	O
management	O
system	O
was	O
met	O
with	O
a	O
negative	O
reaction	O
from	O
PC	B-Device
server	O
operators	O
,	O
who	O
,	O
with	O
this	O
system	O
,	O
had	O
very	O
little	O
control	O
over	O
their	O
servers	O
.	O
</s>
<s>
Valve	O
chose	O
an	O
intro	O
movie	O
over	O
in-game	O
training	O
mechanics	O
because	O
they	O
wanted	O
the	O
players	O
to	O
be	O
immediately	O
dropped	O
into	O
a	O
zombie	B-Application
apocalypse	O
.	O
</s>
<s>
Valve	O
later	O
detailed	O
in	O
their	O
official	O
Left	B-Application
4	I-Application
Dead	I-Application
blog	O
how	O
they	O
designed	O
the	O
movie	O
,	O
from	O
a	O
basic	O
animatic	O
in	O
July	O
2008	O
to	O
the	O
final	O
result	O
for	O
the	O
launch	O
of	O
the	O
game	O
.	O
</s>
<s>
Early	O
access	O
to	O
the	O
Left	B-Application
4	I-Application
Dead	I-Application
demo	I-Application
for	O
people	O
who	O
pre-ordered	O
the	O
game	O
began	O
on	O
November	O
6	O
,	O
2008	O
,	O
for	O
both	O
Windows	O
and	O
Xbox	B-Operating_System
360	I-Operating_System
.	O
</s>
<s>
This	O
promotion	O
was	O
being	O
offered	O
in	O
addition	O
to	O
the	O
ten	O
percent	O
savings	O
for	O
those	O
who	O
pre-order	O
and	O
applies	O
to	O
all	O
Steam	B-Application
Windows	O
pre-orders	O
and	O
all	O
Windows	O
and	O
Xbox	B-Operating_System
360	I-Operating_System
pre-orders	O
from	O
GameStop	O
and	O
EB	O
Games	O
in	O
North	O
America	O
.	O
</s>
<s>
On	O
November	O
11	O
,	O
the	O
Left	B-Application
4	I-Application
Dead	I-Application
demo	I-Application
was	O
made	O
available	O
to	O
all	O
Windows	O
and	O
Xbox	B-Operating_System
360	I-Operating_System
gamers	O
worldwide	O
.	O
</s>
<s>
The	O
Left	B-Application
4	I-Application
Dead	I-Application
demo	I-Application
was	O
removed	O
from	O
the	B-Operating_System
Xbox	I-Operating_System
360	I-Operating_System
Marketplace	O
and	O
Valve	O
's	O
own	O
Steam	B-Application
after	O
the	O
game	O
's	O
release	O
on	O
November	O
18	O
,	O
2008	O
,	O
however	O
it	O
is	O
still	O
available	O
for	O
those	O
that	O
played	O
it	O
.	O
</s>
<s>
On	O
May	O
1	O
,	O
2009	O
,	O
the	O
game	O
was	O
released	O
freely	O
via	O
Steam	B-Application
as	O
a	O
one-day	O
trial	O
called	O
"	O
Freaky	O
Free	O
Friday	O
"	O
.	O
</s>
<s>
On	O
October	O
5	O
,	O
2010	O
,	O
the	O
price	O
of	O
the	O
game	O
via	O
Steam	B-Application
was	O
dropped	O
to	O
US	O
$6.80	O
,	O
or	O
a	O
"	O
4-pack	O
"	O
for	O
$20.40	O
,	O
as	O
a	O
promotion	O
coinciding	O
with	O
the	O
release	O
of	O
"	O
The	O
Sacrifice	O
"	O
DLC	B-Protocol
.	O
</s>
<s>
Left	B-Application
4	I-Application
Dead	I-Application
went	O
gold	O
on	O
November	O
13	O
,	O
2008	O
,	O
and	O
was	O
released	O
on	O
November	O
18	O
,	O
2008	O
,	O
in	O
North	O
America	O
;	O
and	O
on	O
November	O
21	O
,	O
2008	O
,	O
in	O
Europe	O
to	O
coincide	O
with	O
the	O
tenth	O
anniversary	O
of	O
the	O
release	O
of	O
Half-Life	B-Application
.	O
</s>
<s>
Valve	O
released	O
a	O
server	O
patch	O
in	O
December	O
2008	O
to	O
stop	O
players	O
on	O
Xbox	B-Operating_System
Live	O
from	O
using	O
PC-style	O
cheats	O
.	O
</s>
<s>
Valve	O
intended	O
to	O
support	O
the	O
PC	B-Device
version	O
of	O
the	O
game	O
through	O
free	O
content	O
updates	O
and	O
the	B-Operating_System
Xbox	I-Operating_System
360	I-Operating_System
version	O
with	O
DLC	B-Protocol
.	O
</s>
<s>
On	O
a	O
podcast	O
by	O
Kotaku	O
,	O
writer	O
Chet	O
Faliszek	O
divulged	O
that	O
an	O
announcement	O
regarding	O
DLC	B-Protocol
for	O
the	O
PC	B-Device
and	O
Xbox	B-Operating_System
360	I-Operating_System
would	O
be	O
released	O
"	O
very	O
soon	O
"	O
,	O
and	O
that	O
the	O
announcement	O
was	O
delayed	O
by	O
the	O
holiday	O
season	O
.	O
</s>
<s>
On	O
February	O
5	O
,	O
2009	O
,	O
Valve	O
released	O
details	O
about	O
the	O
upcoming	O
downloadable	B-Protocol
content	I-Protocol
pack	O
.	O
</s>
<s>
On	O
February	O
11	O
,	O
2009	O
,	O
Valve	O
announced	O
that	O
the	O
downloadable	B-Protocol
content	I-Protocol
for	O
the	O
game	O
would	O
be	O
free	O
for	O
both	O
the	B-Operating_System
Xbox	I-Operating_System
360	I-Operating_System
and	O
PC	B-Device
;	O
and	O
on	O
April	O
21	O
,	O
it	O
was	O
released	O
.	O
</s>
<s>
A	O
Game	O
Of	O
the	O
Year	O
Edition	O
of	O
Left	B-Application
4	I-Application
Dead	I-Application
was	O
released	O
on	O
the	O
PC	B-Device
and	O
Xbox	B-Operating_System
360	I-Operating_System
on	O
May	O
12	O
,	O
2009	O
,	O
with	O
updates	O
and	O
new	O
content	O
included	O
on	O
the	O
disc	O
.	O
</s>
<s>
On	O
May	O
15	O
,	O
2009	O
,	O
an	O
open	O
beta	O
test	O
for	O
the	O
Source	B-Application
Development	O
Kit	O
updated	O
to	O
support	O
Left	B-Application
4	I-Application
Dead	I-Application
was	O
started	O
under	O
the	O
name	O
of	O
the	O
"	O
Left	B-Application
4	I-Application
Dead	I-Application
Authoring	O
tools	O
"	O
.	O
</s>
<s>
This	O
included	O
a	O
new	O
set	O
of	O
plugins	O
that	O
allowed	O
for	O
users	O
to	O
import	O
data	O
from	O
SketchUp	B-Application
,	O
a	O
free	O
3D	O
modeling	O
program	O
,	O
directly	O
into	O
the	O
Hammer	O
level	O
editor	O
for	O
use	O
in	O
maps	O
.	O
</s>
<s>
The	O
beta	O
was	O
concluded	O
on	O
June	O
25	O
,	O
2009	O
,	O
with	O
the	O
full	O
release	O
of	O
the	O
Left	B-Application
4	I-Application
Dead	I-Application
authoring	O
tools	O
and	O
corresponding	O
server	O
and	O
matchmaking	O
update	O
to	O
support	O
custom	O
maps	O
.	O
</s>
<s>
The	O
update	O
included	O
a	O
command	O
line	O
tool	O
for	O
packaging	O
custom	O
Left	B-Application
4	I-Application
Dead	I-Application
campaigns	O
to	O
ease	O
distribution	O
.	O
</s>
<s>
A	O
sequel	B-Application
,	O
announced	O
at	O
the	O
2009	O
E3	O
conference	O
and	O
was	O
released	O
November	O
17	O
,	O
2009	O
.	O
</s>
<s>
Addressing	O
concerns	O
voiced	O
by	O
fans	O
,	O
Gabe	O
Newell	O
responded	O
to	O
an	O
email	O
from	O
Kotaku	O
explaining	O
that	O
despite	O
the	O
upcoming	O
sequel	B-Application
,	O
Left	B-Application
4	I-Application
Dead	I-Application
would	O
continue	O
to	O
be	O
supported	O
and	O
more	O
content	O
is	O
planned	O
in	O
the	O
coming	O
months	O
.	O
</s>
<s>
On	O
August	O
4	O
,	O
2009	O
,	O
Valve	O
announced	O
the	O
second	O
DLC	B-Protocol
pack	O
.	O
</s>
<s>
It	O
contains	O
a	O
new	O
campaign	O
called	O
Crash	O
Course	O
,	O
set	O
shortly	O
after	O
the	O
events	O
of	O
the	O
No	O
Mercy	O
campaign	O
,	O
where	O
the	O
Survivors	O
try	O
to	O
get	O
to	O
a	O
Truck	O
Depot	O
after	O
the	O
helicopter	B-Application
they	O
were	O
in	O
crashed	O
.	O
</s>
<s>
The	O
DLC	B-Protocol
was	O
announced	O
to	O
be	O
released	O
on	O
September	O
29	O
,	O
2009	O
,	O
on	O
which	O
date	O
it	O
was	O
released	O
for	O
free	O
for	O
PC	B-Device
,	O
but	O
was	O
accidentally	O
released	O
on	O
Xbox	B-Operating_System
Live	O
at	O
a	O
higher	O
price	O
.	O
</s>
<s>
The	O
price	O
was	O
amended	O
soon	O
after	O
,	O
and	O
all	O
players	O
who	O
bought	O
the	O
DLC	B-Protocol
at	O
the	O
higher	O
price	O
were	O
refunded	O
.	O
</s>
<s>
An	O
add-on	O
campaign	O
for	O
Left	B-Application
4	I-Application
Dead	I-Application
2	I-Application
,	O
"	O
The	O
Passing	O
"	O
,	O
featured	O
the	O
Left	B-Application
4	I-Application
Dead	I-Application
survivors	O
meeting	O
the	O
new	O
cast	O
as	O
part	O
of	O
a	O
full	O
campaign	O
.	O
</s>
<s>
The	O
Left	B-Application
4	I-Application
Dead	I-Application
2	I-Application
content	O
was	O
set	O
for	O
release	O
in	O
March	O
2010	O
,	O
however	O
it	O
was	O
delayed	O
until	O
April	O
22	O
,	O
2010	O
.	O
</s>
<s>
To	O
connect	O
the	O
events	O
in	O
"	O
The	O
Passing	O
"	O
to	O
that	O
in	O
Left	B-Application
4	I-Application
Dead	I-Application
,	O
Valve	O
released	O
another	O
add-on	O
in	O
October	O
2010	O
for	O
both	O
Left	B-Application
4	I-Application
Dead	I-Application
and	O
its	O
sequel	B-Application
,	O
entitled	O
"	O
The	O
Sacrifice	O
"	O
;	O
the	O
events	O
of	O
the	O
add-on	O
take	O
place	O
before	O
"	O
The	O
Passing	O
"	O
and	O
tells	O
how	O
the	O
Left	B-Application
4	I-Application
Dead	I-Application
survivors	O
come	O
to	O
encounter	O
the	O
group	O
from	O
Left	B-Application
4	I-Application
Dead	I-Application
2	I-Application
,	O
and	O
how	O
one	O
sacrificed	O
themselves	O
for	O
the	O
safety	O
of	O
the	O
others	O
.	O
</s>
<s>
In	O
The	O
Sacrifice	O
DLC	B-Protocol
,	O
any	O
player	O
,	O
whether	O
playing	O
as	O
Bill	O
or	O
not	O
,	O
can	O
choose	O
to	O
sacrifice	O
themselves	O
to	O
allow	O
the	O
other	O
survivors	O
to	O
complete	O
the	O
campaign	O
.	O
</s>
<s>
In	O
March	O
2010	O
,	O
Valve	O
announced	O
that	O
it	O
would	O
be	O
bringing	O
the	O
Steam	B-Application
content	O
platform	O
to	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
computers	O
;	O
along	O
with	O
it	O
,	O
they	O
will	O
be	O
providing	O
native	O
versions	O
of	O
existing	O
Valve	O
games	O
including	O
Left	B-Application
4	I-Application
Dead	I-Application
and	O
its	O
sequel	B-Application
.	O
</s>
<s>
The	O
game	O
was	O
released	O
for	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
on	O
October	O
27	O
,	O
2010	O
.	O
</s>
<s>
Left	B-Application
4	I-Application
Dead	I-Application
support	O
cross-platform	O
play	O
,	O
allowing	O
Mac	O
players	O
to	O
play	O
alongside	O
PC	B-Device
players	O
on	O
the	O
same	O
servers	O
,	O
and	O
is	O
also	O
part	O
of	O
the	O
"	O
Steam	B-Application
Play	O
"	O
cross-compatible	O
and	O
Steam	B-Application
Cloud	O
titles	O
,	O
allowing	O
a	O
player	O
that	O
has	O
purchased	O
the	O
game	O
on	O
one	O
platform	O
to	O
download	O
and	O
play	O
it	O
on	O
the	O
other	O
platform	O
for	O
free	O
.	O
</s>
<s>
Overkill	O
Software	O
,	O
the	O
developers	O
of	O
Payday	B-Application
:	I-Application
The	I-Application
Heist	I-Application
,	O
a	O
similar	O
game	O
to	O
Left	B-Application
4	I-Application
Dead	I-Application
featuring	O
four	O
player	O
co-operative	O
gameplay	O
,	O
announced	O
in	O
June	O
2012	O
that	O
it	O
has	O
worked	O
with	O
Valve	O
to	O
create	O
a	O
DLC	B-Protocol
level	O
for	O
Payday	B-Application
in	O
the	O
form	O
of	O
a	O
map	O
mirroring	O
the	O
"	O
No	O
Mercy	O
"	O
level	O
from	O
Left	B-Application
4	I-Application
Dead	I-Application
,	O
however	O
,	O
this	O
DLC	B-Protocol
level	O
is	O
not	O
canon	O
to	O
Left	B-Application
4	I-Application
Dead	I-Application
and	O
does	O
not	O
lead	O
to	O
the	O
infection	O
that	O
initiated	O
the	O
events	O
in	O
Left	B-Application
4	I-Application
Dead	I-Application
.	O
</s>
<s>
On	O
August	O
20	O
,	O
2015	O
,	O
an	O
update	O
for	O
the	O
game	O
Zombie	B-Application
Army	I-Application
Trilogy	I-Application
was	O
released	O
,	O
which	O
imported	O
the	O
eight	O
survivors	O
from	O
both	O
Left	B-Application
4	I-Application
Dead	I-Application
games	O
into	O
ZAT	O
.	O
</s>
<s>
Downloadable	B-Protocol
content	I-Protocol
for	O
Zombie	B-Application
Army	I-Application
4	I-Application
:	I-Application
Dead	I-Application
War	I-Application
in	O
2021	O
included	O
the	O
four	O
Left	B-Application
4	I-Application
Dead	I-Application
survivors	O
as	O
playable	O
characters	O
as	O
part	O
of	O
its	O
co-op	O
mode	O
.	O
</s>
<s>
The	O
character	O
of	O
Bill	O
was	O
introduced	O
in	O
the	O
game	O
Dead	B-Application
by	I-Application
Daylight	I-Application
in	O
that	O
game	O
's	O
DLC	B-Protocol
in	O
2017	O
,	O
and	O
was	O
given	O
an	O
expanded	O
lore	O
scenario	O
around	O
his	O
time	O
in	O
the	O
Vietnam	O
War	O
in	O
additional	O
content	O
released	O
in	O
2021	O
.	O
</s>
<s>
At	O
Metacritic	O
,	O
the	O
game	O
received	O
an	O
aggregated	O
score	O
of	O
89	O
out	O
of	O
100	O
for	O
both	O
Xbox	B-Operating_System
360	I-Operating_System
and	O
PC	B-Device
.	O
</s>
<s>
IGN	O
stated	O
,	O
"	O
It	O
's	O
almost	O
pitch	O
perfect	O
in	O
how	O
it	O
captures	O
the	O
tension	O
and	O
the	O
action	O
of	O
a	O
Hollywood	O
zombie	B-Application
movie	O
"	O
,	O
and	O
went	O
on	O
to	O
describe	O
it	O
as	O
,	O
"	O
quite	O
possibly	O
the	O
perfect	O
co-op	O
shooter.	O
"	O
</s>
<s>
Giant	O
Bomb	O
commented	O
that	O
the	O
Source	B-Application
engine	I-Application
was	O
beginning	O
to	O
show	O
its	O
age	O
,	O
but	O
praised	O
the	O
game	O
's	O
use	O
of	O
lighting	O
and	O
filmic	O
effects	O
that	O
gives	O
the	O
game	O
world	O
,	O
"	O
a	O
desolate	O
,	O
washed-out	O
feeling	O
"	O
,	O
as	O
well	O
as	O
the	O
realistic	O
and	O
emotive	O
faces	O
and	O
the	O
engrossing	O
art	O
direction	O
.	O
</s>
<s>
Eurogamer	O
concluded	O
that	O
Left	B-Application
4	I-Application
Dead	I-Application
"	O
is	O
another	O
deeply	O
professional	O
,	O
personality-filled	O
and	O
progressive	O
take	O
on	O
the	O
shooter	O
from	O
Valve.	O
"	O
</s>
<s>
However	O
,	O
some	O
reviewers	O
praised	O
its	O
faithfulness	O
to	O
the	O
zombie	B-Application
film	O
genre	O
,	O
including	O
the	O
"	O
deliberately	O
ambiguous	O
"	O
back-story	O
,	O
and	O
the	O
amount	O
of	O
characterization	O
and	O
emotion	O
brought	O
by	O
each	O
of	O
the	O
four	O
survivors	O
.	O
</s>
<s>
TeamXbox	O
commented	O
that	O
clipping	B-Algorithm
issues	O
hurt	O
the	O
otherwise	O
"	O
incredibly	O
good	O
"	O
visual	O
experience	O
.	O
</s>
<s>
Hideo	O
Kojima	O
,	O
creator	O
of	O
the	O
Metal	B-Application
Gear	I-Application
series	I-Application
declared	O
in	O
an	O
interview	O
with	O
1UP.com	O
that	O
he	O
was	O
"	O
addicted	O
to	O
the	O
game	O
"	O
,	O
which	O
was	O
,	O
in	O
his	O
view	O
,	O
one	O
of	O
the	O
"	O
core	O
titles	O
made	O
with	O
movie-industry	O
people	O
that	O
explore	O
the	O
depths	O
of	O
hi-def	O
"	O
.	O
</s>
<s>
On	O
October	O
28	O
,	O
2008	O
,	O
Valve	O
reported	O
that	O
preorders	O
for	O
Left	B-Application
4	I-Application
Dead	I-Application
had	O
beaten	O
those	O
of	O
The	B-Application
Orange	I-Application
Box	I-Application
by	O
95	O
percent	O
after	O
the	O
Steam	B-Application
pre-order	O
was	O
launched	O
.	O
</s>
<s>
On	O
November	O
21	O
,	O
2008	O
,	O
the	O
day	O
of	O
the	O
game	O
's	O
release	O
in	O
Europe	O
,	O
Valve	O
issued	O
a	O
press	O
release	O
stating	O
that	O
Left	B-Application
4	I-Application
Dead	I-Application
had	O
exceeded	O
the	O
pre-order	O
numbers	O
of	O
The	B-Application
Orange	I-Application
Box	I-Application
by	O
over	O
160	O
percent	O
.	O
</s>
<s>
The	B-Operating_System
Xbox	I-Operating_System
360	I-Operating_System
version	O
of	O
Left	B-Application
4	I-Application
Dead	I-Application
was	O
the	O
seventh	O
best-selling	O
game	O
of	O
December	O
2008	O
in	O
the	O
United	O
States	O
,	O
selling	O
in	O
excess	O
of	O
629,000	O
copies	O
.	O
</s>
<s>
On	O
February	O
3	O
,	O
Electronic	O
Arts	O
revealed	O
that	O
Left	B-Application
4	I-Application
Dead	I-Application
had	O
sold	O
1.8	O
million	O
copies	O
,	O
excluding	O
Steam	B-Application
and	O
worldwide	O
sales	O
figures	O
.	O
</s>
<s>
On	O
May	O
10	O
,	O
2011	O
,	O
Doug	O
Lombardi	O
mentioned	O
that	O
the	O
game	O
and	O
its	O
sequel	B-Application
have	O
each	O
sold	O
3	O
million	O
copies	O
on	O
the	B-Operating_System
Xbox	I-Operating_System
360	I-Operating_System
.	O
</s>
<s>
AwardsLeft	O
4	O
Dead	O
received	O
recognition	O
as	O
one	O
of	O
the	O
best	O
multiplayer	O
and	O
PC	B-Device
games	O
of	O
2008	O
from	O
various	O
organizations	O
and	O
gaming	O
publications	O
.	O
</s>
<s>
Other	O
awards	O
include	O
"	O
Outstanding	O
Achievement	O
in	O
Online	O
Gameplay	O
"	O
from	O
the	O
AIAS	O
,	O
Best	O
Use	O
of	O
Sound	O
for	O
the	O
PC	B-Device
and	O
Best	O
Shooting	O
game	O
overall	O
from	O
IGN	O
,	O
Father	O
of	O
All	O
FPS	O
from	O
Nofrag	O
,	O
and	O
the	O
best	O
Cooperative	O
Multiplayer	O
and	O
Shooter	O
of	O
2008	O
from	O
GameSpot	O
who	O
also	O
nominated	O
it	O
for	O
Game	O
of	O
the	O
Year	O
.	O
</s>
<s>
LegacyLeft	O
4	O
Dead	O
and	O
its	O
sequel	B-Application
created	O
a	O
format	O
for	O
several	O
games	O
that	O
followed	O
that	O
used	O
the	O
same	O
four-player-versus-environment	O
style	O
of	O
gameplay	O
using	O
artificial	B-Application
intelligence	I-Application
similar	O
to	O
the	O
AI	B-Application
Director	O
and	O
encourages	O
strong	O
cooperation	O
between	O
players	O
.	O
</s>
<s>
Such	O
Left	O
4	O
Dead-likes	O
,	O
according	O
to	O
Polygons	O
Josh	O
Rios	O
,	O
included	O
games	O
like	O
Warhammer	B-Application
:	I-Application
End	I-Application
Times	I-Application
–	I-Application
Vermintide	I-Application
and	O
its	O
sequel	B-Application
,	O
and	O
Deep	B-Application
Rock	I-Application
Galactic	I-Application
,	O
but	O
only	O
about	O
a	O
decade	O
after	O
Left	B-Application
4	I-Application
Deads	I-Application
release	O
did	O
a	O
large	O
number	O
of	O
titles	O
in	O
this	O
genre	O
emerge	O
.	O
</s>
<s>
Seven	O
of	O
the	O
original	O
Left	B-Application
4	I-Application
Dead	I-Application
developers	O
from	O
Turtle	O
Rock	O
Studios	O
were	O
part	O
of	O
the	O
team	O
that	O
released	O
a	O
spiritual	O
successor	O
titled	O
Back	B-Application
4	I-Application
Blood	I-Application
in	O
October	O
2021	O
.	O
</s>
<s>
Several	O
Left	O
4	O
Dead-like	O
games	O
were	O
announced	O
ahead	O
and	O
during	O
E3	O
2021	O
,	O
including	O
Rainbow	B-Application
Six	I-Application
Extraction	I-Application
,	O
The	O
Anacrusis	O
,	O
Redfall	B-Application
,	O
Warhammer	B-Application
40,000	I-Application
:	I-Application
Darktide	I-Application
and	O
Evil	B-Application
Dead	I-Application
:	I-Application
The	I-Application
Game	I-Application
.	O
</s>
