On a side note, the Cordyceps are a REAL existing variety of fungi. Aproximately 400 fungi, which all spread through mainly their spores. Once a host is infected, the fungus takes control over the brain and forces the host to infect others via biting. With time it spreads so far that it grows out through the eyes and becomes lastly visible for what it is. At this point most hosts are close to death and the cordyceps will make them search for a good spot, somewhere high up, where they will die. There the fungus can grow big enough to spread more spores.

So far successful spread has only been seen on insects and closely related species. It has however proven to be able to mutate. It has already infected other fungi and even small rodents. Yet those infections have not nearly been as successful and lead to almost immediate death of the host, which did not give said Cordyceps enough time to grow.

To save time, I took this straight (with formatting alterations) off the Wikia:

As the infection progresses, the infection begins altering their sight as a result of progressing fungal growth over the head and corruption of their visual cortex.

Stage 2 starts anywhere in between 1-2 weeks of infection. The infection will eventually scar their face completely, causing them to lose their sight, resulting in them developing a primitive form of echolocation to compensate.

Stage 3 starts anywhere in between 1-2 years of infection. Over a very long time, they will eventually develop hardened fungal plates over most of their body.

Stage 4 starts anywhere in between 1-2 decades of infection. If the fungus kills the host, the host's body will grow stalk-like fungal projections which release infectious spores. The infection can also be spread through bites from living hosts. Hosts can only be infected while alive, as the fungus is unable to infect dead bodies due to it's parasitic nature, though dead Infected can release spores regardless of stage.

This is therefore not far from the already existing fungus. The scenario however, aaaaaahm - shall I say, "Please don't panic!"?

In the game, we start in the suburb of Austin, Texas, where the girl Sarah is woken in the night of her dad's; Joel; birthday by a call of her uncle Tommy. He sounds frightened, but the connection broke. So she goes downstairs only to find the TV turned on with the running news of infection increases and a crisis in Austin's city. An explosion happens right in front of the camera and Sarah finally is awake, as she sees it happen through the window, against Austin's skyline.

Scared she progresses through the house to the ground floor, where finally her father bursts in through the veranda door, desperately looking for his gun while trying to calm his confused daughter.

This is where the infection hits also Sarah's consciousness in shape of the completely deranged and blood-soaken neighbour bursting through the closed glass door..

[..don't want to spoil too much here..]

Joel now makes his living as a varied smuggler in the military controlled quarantine zone of Boston, along with his partner Tess.

One day he wakes up from a nightmare, only to find Tess coming back from a failed contract. She had been ambushed by the minions of an arms dealer named Robert. They go looking for him, finding out that the reason for the ambush was that the weapons he had promised them, he had also promised to Marlene, the leader of the radical resistance group Fireflies.

Upon killing Robert in the course of the interrogation in a backyard, they have to face Marlene, who had also noticed Robert's far more extensive betrayal, though had come too late for her own revenge.

Both parties having been betrayed, they understand that the only way is a deal: weapons for cargo. Marlene offers them to hand over the weapons, in exchange for the two to smuggle something out of the quarantine zone and to the former Boston town hall for her.

  It turns out the be a wannabe reckless teenage girl, Ellie, who has never seen anything but the safe quarantine zone..

However this happens to be a lie sort of. In course of their little dangerous trip through broken in sewerage past the high concrete walls around the zone and collapsed skyscrapers, she confesses that she and her friend Riley had sometimes snuck into condemned buildings, if though only within the zone, to find out more about the story behind the mysterious infected and why everyone entering such a building was shot immediately upon their leave when getting caught.

One day, both got bitten. Desperate, Ellie turned to Marlene, a former friend of her deceased mother and much like an aunt to her. Marlene luckily realised the situation: Riley's visible infection progressed much faster, while Ellie's bite healed off like a sort of acid burn. She survived weeks past Riley - something never seen before. Ellie's body has successfully fought the Cordyceps. She is immune.

  Finally reaching the town hall after several encounters with infected in various states, the military has gotten wind of their escape and force them inside, where they find the Fireflies who were to pick up Ellie: dead. Worse, Tess sees no other option than to reveal that she has been bitten in one of the ruins. Certain that Ellie's story is real since her own injury already looks worse, she persuades Joel to go on with Ellie and look for his brother Tommy; now living in Wyoming; who used to be a Firefly himself..