Green Flu

Like many of the more recent incarnations of zombie movies, videogames, and books, the Infected are zombie-like creatures caused by an unknown, as-yet-unnamed infection. There has been much speculation as to the cause of the infection, or whether or not its origin will even have any relevance at all in the plot of the game (especially if, as some have speculated, each campaign of the game is a separate plotline).

Valve games typically contain stories that are expressed through exploring the world and interacting with its inhabitants rather than from watching lengthy cut-scenes or reading explanatory paragraphs out of the game manuals. From what we've seen of the game so far, this seems to be the case in Left 4 Dead as well. Graffiti covers many walls of the game, often containing messages to separated loved ones, foreboding messages of warning, and, most interestingly, past survivors' own explanations for the infection.

Means of infection
The intro movie to the game begins with the cryptic statement "2 WEEKS AFTER FIRST INFECTION," implying that a single individual was once infected with an unknown virus of some sort, and the virus spread from there. What's not immediately clear, however, is the means of spreading the infection. The infected don't seem necessarily ready to bite players, as other more "classical" zombie viruses spread.

However, immediately after the aforementioned intro card for the opening cut scene, Bill is seen discovering a viscous, bright green substance in or on (it's difficult to discern) a deceased (and presumably infected) corpse. He mentions that he hasn't ever seen anything like it before, but it doesn't seem to hurt him in any immediate way, and neither does smearing it on Francis' jacket.