The end is nigh. The infected are lurking everywhere and chances of survival are slim. Left 4 Dead focuses on the story of 4 individuals and their harrowing battles against the hordes of the infected that roam the streets. What about other survivors? What would they look like and what kind of deformed monstrosities would they be up against?
When the infection was unleashed, it spreaded like burning powder. People fell ill without a clear reason. Suffering from pain and fearing the unknown, they started to change... to mutate.
People were transformed into mindless zombie-like living beings. Wild beings, living to eat. Never resting. Never reasoning. Just instinct. Survival instinct. Worse than animals.
Soon afterwards it became clear that not only “normal” mutated people had appeared. There were sightings of stranger (and more dangerous) creatures, known by the survivors as “bosses”. These infected were tougher and had abilities that surpassed those of the usual infected. The infection was produced by what the scientists called “a virus”; a nice and scientifical way to say that they didn't know what it was...
The virus changes the metabolism of the host's body. Mutates and reproduces inside it, producing changes in the cells and modifying vital processes.
The infection has an incubation period, during which it would be possible to cure the patient (assuming a cure was found). The period can vary in length depending on the host's body resistance, kind of mutation, climatological factors, etc...
The virus commonly infects and mutates the host in stages. Each stage is a transformation process of a kind. The whole transformation process takes place after a certain number of stages; the number and type of the stages determines the type of infected that will be produced. Mutations of the virus can produce new stages, and thus, new infected types.