Okay, the green flu is the name given by the government to the virus in Left 4 dead. So named probably because "flu" isn't that alarming a term...when there was much to be alarmed about. The infection period is rapid. In the sacrifice comic, it seems to be sometimes instant, but evidence of prolonged infected prescence at mercy hospital and writings left by survivors debating the infection period seems to suggest that it may be varied, which only makes containing the virus even more difficult. Imagine a virus for which there is no certain incubation period. No certain period after which an infected area can be declared "safe". To contain such a virus, a policy of abandonment of all survivors could be the only hope.
An even more disturbing nugget of information comes from the comic; According to a doctor, the virus changes on an almost daily basis. "One day it's not airborne, the next day it is" To put this in practical terms, the pile of the dead outside your stronghold is no threat one day...the next, it infects even just person inside, dooming you all.
There is only one hope for a cure...but they are also perhaps the biggest threat to containment. Some people are immune. The survivors from L4D1/2 are such people. To most, one bite, scratch, contact, or even near proximity to infected without an envirosuit or gasmask can be the death of them. However, immunes can sustain all kinds of wounds from infected, and still fight on as long as their heart is beating. Or rather, intact and uneaten. The problem is that they are not only immune, they are potential carriers. Infected contact will still infect, just with no symptoms or "health complications" whatsoever. Contact with them could in fact guarantee infection. Thus the reason for the survivors bad luck with rescue attempts. As long as their bodies are living and undestroyed, they will continue to carry the virus.
Now onto the really bad part; what becomes of the infected. Initial symptoms do indeed seem flu-like. Vomiting, sweating, the normal stuff. When the infected turn, the primary effect is that they become hostile to all normal humans. Perhaps some resistance to this exists, as suggested by the fallen survivors in the passing dlc, but eventually, and inevitably, all infected that are not immune lose all control over themselves, becoming locked in a state of rage, beating and biting all humans they come across. They also react to loud noise and lights, chasing them down in search of prey. The infected also seem to hemorage (sp?) at this stage, blood coming from the eyes, ears, and even vomited up, causing them to spread the infection even more effectively.
Elimination is at least simple. They seem to have a limited ignorance of damage done to themselves. They will only notice major wounds. Even them, some infected have been known to pursue humans after the removal of an arm. Luckily, headshots, although effective, are not required. They may ignore damage, but thankfully are killable with a few bullets just as normal humans are.
As for intelligence, they are mostly stupid, but some have shown limited improvisation skills, as evidenced by some using muddy swamp waters for cover and then popping up out of the water to devour unlucky passers by.
The worse part of the infection and the single greatest risk to containment are the "special" infected. These are normal infected mutated into several known deadly forms. When and how this happens is debatable. It seems it is an effect of the same daily mutation of the virus itself the doctor from the comic mentioned. It seems to be mostly an effect that occurs after infected turn, but there is an exception in the person in the church in the death toll campaign, that seems to go from human to boomer in a matter of seconds.
There are the hunters, capable of jumping over any barricade, and jumping to eviscerate survivors. Jockeys, who excel at jumping on survivors and using their momentum to lead them into deeper danger. Smokers that pull humans towards itself using a long tongue, and emit an obscuring cloud when killed. Boomers that vomit no-doubt infected bile which attracts normal infected. Witches that deceptively cry and then attack anyone who comes near with deadly efficiency. Spitters that spit acid, chargers that scatter groups and pummel those they grab to their deaths. Finally, there is the tank. Huge, immensely strong behemoths, capable of ripping up pieces of rock and tossing it at humans. This tactic is usually used to slow humans down, so tanks can move in on their victims.
Together, these infected almost form a team; a set of different threats, capable of negating seemingly any physical barricade put up to contain the virus, and taking down almost any military countermeasures in their way.
The green flu virus, in summary, is the perfect storm of virus', Virtually uncontainable, and almost certainly uncurable. To show this: According to the comic, pennslyvania was in a state of chaos within the first 2 days since patient zero. The time in between then and the comfirmed destruction of new orleans in left 4 dead 2 was around three weeks.