Infected humans that hunt you by clicking.
The office building was slanted sideways, as if it had been frozen in the middle of falling over. The three of them could see lighting flash through the holes in its concrete frame. They climbed over rubble in the streets and entered it through a broken window. It was a huge risk, but they had to reach the capitol building as soon as possible and this was the fastest way they could get through the downtown area. Inside, plants grew across walls and doors and water from the storm flowed through cracks in the ceiling. It was pitch black so they turned on their flashlights. That's when they saw the first body. A soldier had been ripped to shreds. What was left of him lay against the wall in the hallway. The blood was still fresh. They quickened their pace. They didn't want to spend a moment longer than necessary in this terrible building.
At the top of a staircase the man found a door blocked by another corpse, though it was hard to recognize it as human. The body sat upright completely covered in some sort of fungus. He could see graceful tendrils climbing up and out of its legs, torso, and arms. Its head was completely split open, the face hidden behind thick layers of mushrooms and old blood. He could barely see the person's mouth underneath the dank skull, the lips pulled back, frozen in a painful grimace. The fungus climbed up and out of the skull and coated the door and walls beside it. "Goddammit. Clicker." he said.
The man peeled the corpse away from the door and went through. The two women followed and they progressed through a series of offices that slanted towards the ground 100 feet below. One of the women, the one in her teens, started to feel sick from the extreme angles and the memory of the fungal corpse. Water from the storm flowed past them on the ground and out the holes gaping in the side of the building. They felt as if the building had tipped even further since they entered it. Finally, they found a door heading back into the hallway but it was stuck. The adults both put their shoulders to it and managed to heave it open, the obstruction screeching along the floor as they did. The woman shouted and the man looked up just as the monster lunged on top of him knocking him to the ground. It was all he could do to prevent it from biting his face off. His vision was filled with the gruesome sight of a divided skull covered in fungus. All he could hear were the screeches and rapid clicking sounds coming from the monster's throat. It was much stronger than any human, and his strength was failing fast. At the last moment, the woman desperately kicked it off of him and put 2 bullets in its head. It stopped moving, but they knew the gunshots would attract more undead. They had to keep moving.
There is a BBC documentary that shows a clip of an ant infected by the Ophiocordyceps unilateralis fungus. The fungus controls its mind, causing erratic behaviour and compelling it to spread spores. The ant is utterly destroyed in the process. These invasive species of fungi are easy to find in the insect world. We normally don't need to worry about them spreading in humans. But the horrifying truth is that there is a rare variant of the Ophiocordyceps unilateralis fungus that has mutated to infect human beings. Its common name is the Cordyceps infection (although Cordyceps is technically a genus of fungi). The footage of the ant gives you a basic idea of its effects. The fungus infects the human brain, slowly taking over the mind of its host and compelling it to attack other people so that it may spread. Eventually, it bursts forth from the host's body so that its spores may spread and infect others.
There are four distinct stages as the infection progresses in a human. Each stage seems less and less human and is more of a threat to others as the fungus forms a sort of protective armor and reinforces the muscle fibers granting super strength. The stages of infected are known colloquially as Runner, Stalker, Clicker, and Bloater or Shambler. The initial phases still seem to retain some of their humanity. You will notice expressions of concern and even terror on their faces, suggesting they may have some awareness of what they are doing even though they can't stop themselves. They move between fits of rage and calm and you can often hear them moaning as if in pain.
After about a year of infection, the infected becomes a Clicker. This is an advanced stage of infection. At this stage, there are almost no remaining traces of humanity. The fungus has burst forth from their skull, splitting it in two. It covers their eyes completely so that they lose their sense of sight. All that remains of the human face is a mouth filled with rotted and jagged teeth. They use a primitive sort of echolocation to navigate their environment. The incessant clicking sound they make gives them their name. There are some drawings and photos of these zombie-like monsters but I caution you: Do not look at them unless you have a hard stomach.
Clickers develop a more advanced predatory intelligence. If they hear a sound in their surrounding area, they will continue to search methodically for prey. They are much stronger than a normal human, and are protected by layers of thick fungus. They still feel pain but have a much higher tolerance than an uninfected human. If you encounter one, you must remember that normal attacks may not stop them immediately. You should prefer high-powered, hard-hitting attacks if you need to confront one. They are very susceptible to fire, given the flammable mycellium that has spread throughout their bodies. Your best bet is to use stealth and steer clear of them entirely. Even if you manage to survive a direct encounter, a single scratch or bite will infect you, and you will feel your own humanity painfully slip away as the fungus spreads throughout your body.
Written by Giles Ravensong.