So I figured the ask would be ENTIRELY too long to explain the L4D zombie infestation, so I’m adding it to your submit box.
Left4Dead’s infection was initially called the Green Flue. A rabies type pathogen that incites humans to extreme violence to pretty much anything they see. CEDA referred to it as a flu to try and quell fear across the US, but by the graffiti scrawled along the walls of safe houses it’s pretty apparent no one believed them and people attempted to flee en mass. In both games and the comic there was no explanation of where it started but there’s vague reference to it possibly being transferred from animals to humans via cows. (In Blood Harvest there are piles of dead cows in an empty barn). The best they were able to tell was that the immunity passed down through the fathers genes.
Though “immunity” might not be the right word to use here since being immune to the zombification doesn’t mean you’re immune to the virus. Essentially the population is divided into people who have never been around the virus (VERY few), zombies (massive) and a small section of people referred to as carriers. Carriers show no outward symptom of the virus but infect anyone without the immunity. Before the government fell carriers were generally killed because they spread the virus silently and were deemed dangerous to the public.
This all happened in about 2 weeks. The first part of the game shows a completely destroyed Philadelphia after 2 weeks of infection, which leads many to believe that on top of a bite infection, the carrier exposure made for intensely rapid spread across America. This we can see from the many maps the survivors come across that show different sections of the country have fallen simultaneously. Too many infected, not enough who aren’t to fight them off.
To top that off? There’s also rapid mutation amongst the infected after only 2 weeks of exposure. You could lend that to them possibly eating each other or infected meat in some ways since they show no respect to preservation of their own kind and will kill each other.
The L4D Wikia gives a general explanation based on in game events to the fall of the government in merely days.
Day 2: At this point it seems most people still think the infection is just another flu and was not that harmful. CEDA got involved and informed the public on ways to prevent the spread of infection. However, it also seems this is the day that infected start appearing on the streets of Philadelphia and began to overrun the city.
▪ Day 7 (assumed): Last time Millhaven MIlitary finds any other Survivors so most of the surounding area has probably been infected by now.
▪ Day 11 (assumed): Last time Millhaven hears anything from command.
▪ Day 14: By this time most of Pennsylvania is overrun, and the infected start mutating into the “Special Infected” and the flu starts heading west and south.
▪ Day 19 (assumed): Millhaven Millitary outpost is overrun due to sounding an alarm.
▪ Day 20 (assumed): By this time and probably earlier, The Infection has reached Georgia, where Rayford and Savannah are located. This is also assumed to be the day such mutations as the Charger, Jockey, and Spitter started appearing in the southern states.
▪ October 8, 2009: The Infection reaches Griffin, Georgia so the Midnight Riders have to cancel a concert and are picked up by a helicopter.
Day 24 (assumed): New Orleans is evacuated because of the infected population, and Veterans Memorial Bridge is bombed to slow down the infection.
In 1 month America loses 50% of it’s population.
Due to the long time it takes for us to make vaccinations a virus like the green flu would kill millions upon millions of people before anyone at the CDC could get a handle on it.