The Zombies
[A recording by a scientist made approximately two years into the apocalypse]
The infected, or "zombies" are the result of an unknown parasite which has taken over the host human's body and turned them into a cannibalistic killing machine. The parasite spreads when an infected individual's bodily fluids such as sweat, blood, or more commonly saliva come into contact with an uninfected person. This means that if fluids from a zombie touch an open wound, then you'll turn. However, it is most often biting that spreads the virus. Seconds after infection, the victim's eyesight will start to dull, their behavior will become erratic and unpredictable, and then they'll collapse and begin to seize, foaming at the mouth as their body violently shakes. This process is what one will experience before their body goes limp and they die before "coming back to life" as a zombie, which is really just the parasite taking over and stimulating the brain.
I've found that each zombie exhibits behaviors similar to those that they did when they were human. Those that had more secluded, introverted personalities while they were alive tend not to attack people as often, while an extrovert might be more aggressive, persistent, and bold in pursuing anything that it views as prey. I've even observed infected kicking around stray soccer balls with their feet. Another common trait that I've observed from the zombies is that they display nervous, erratic body language— oftentimes, I spot them chewing on their fingernails, pacing, and frequently looking behind themselves.
Unlike those seen in movies, the zombies in this world show signs of intelligence. Whether it be memorizing faces, remembering the direction that a potential meal ran in, or even just climbing over an obstacle— these things are much more terrifying than anything from a zombie film. You can't just hide behind a fence to avoid them, instead, you'll have to think if you want to survive an encounter with one of the infected. On occasion, they can even use the voices that they once had when they were human, and will use that to lure a gullible survivor into an isolated area. Talk about creepy.
Thankfully, zombies are rather impartial to light, and will prefer to stay in dark, shaded areas during the daytime. On the contrary, they tend to be extremely active during the night, as that's when they're on the hunt for a potential meal. Though, their eyesight is extremely poor, which actually isn't much of a detriment when you consider how good their senses of hearing and smell are. These creatures are able to pick up the scent of blood from several meters away, and can hear even the most subtle of sounds. If you find yourself in a dangerous situation with a zombie, then consider using sound to lure them away. There isn't much you can do about your scent, but, with how sensitive the infecteds' ears are, sound is the best option for escape.
I've organized what "class" a zombie belongs to based on factors such as intelligence, physical strength, and how developed their senses are, with the first class being the most dangerous and the third being the easiest to deal with.
Class 3: Threes/Thirds
Thirds, which are the lowest, most common class of zombie, also happen to be the easiest to dispose of. They are quickly identifiable, even if you aren't able to get a close look at one. They drag their feet, hunch over when they walk, and let out loud, audible groans and growls instead of coherent speech. If you can manage to get close to a Third, then you'll find that their eyes are soulless and harbor no emotion— instead, they're pure white without any iris or pupil. Thirds, like all other zombies, smell strongly of death and rotting flesh.
Thirds are the class with the least physical strength and intellectual abilities. They are no more athletic than they were when they were human— in fact, some have even deteriorated in terms of physical fitness. Their intelligence levels range from being able to memorize faces to knowing how to avoid simple obstacles such as fences, as I described earlier. However, staying inside behind a locked door should keep you safe from a Third, since, strangely enough, they don't seem to be aware of how to unlock them. A horde, however, won't waste any time in destroying a weak door if they can just take it down with sheer strength in numbers. They lose interest in survivors that've run away rather quickly.
If you find yourself in a scuffle with a Third, then you'll find that killing them is quite the easy task. They have predictable, simple attack patterns and only about as much athleticness as a normal person. Simply aim for their brain, heart, or lungs and you'll have yourself a dead Third. Destroying major blood vessels, organs, or depleting them of too much blood will also result in their death. You can also incapacitate them by inflicting significant enough damage to their limbs, preferably their legs.
Assessment: Not a threat in small numbers— a skilled survivor can kill or incapacitate.
Class 2: Twos/Seconds
Unlike Thirds, Seconds, on the other hand, are much more dangerous. They aren't as common as Thirds, but, for the most part, look and act similarly, which makes them difficult to tell apart. The differences in physical appearance and body language between the two classes are extremely subtle— Twos lift their feet slightly higher than Thirds when they walk and keep their backs slightly straighter instead of completely slouching over. Occasionally, they may let out a snarl, but aren't as vocal as Threes. The most major difference is that a Two's eyes are purely black in color instead of purely white like a Third's. While Seconds still do harbor small pieces of their personalities from when they were once human, they are much more anxious and unpredictable in behavior than Thirds. This makes them significantly more aggressive and prone to attack.
The real danger lies in a Second's level of intelligence. They can do anything that a Three can, but better. They know not just how to climb over an obstacle, but can find shortcuts around them. They can memorize faces and names that they hear, and comprehend basic speech between people. In fact, they can even form simple sentences themselves and mimic the voices that they once had as humans. They will use their voices to yell out for help, luring in an unfortunate survivor to a secluded place before going for the kill. Additionally, Seconds know how to open doors, and if they're unable to open it and there's a ventilation shaft around, then they'll climb through vents until they reach the room in which they want to get into. They do not lose interest in prey as easily as a Third will.
A Two can't be killed as easily as a Three— in fact, they're moderate to take down in terms of difficulty. They have enhanced physical strength and speed compared to when they were human, and better hearing and smell than Thirds, although their eyesight is just as terrible. They have less predictable, more erratic attack patterns, making them hard to hit. Their skin is tougher than a Third's as well, so trying to cut off limbs or pierce through their lungs, heart, or organs might require some effort. The quickest way to kill them is to aim for their head— a well-placed shot to the brain is enough to render them dead. If you want to run from one, keep in mind that they won't fall for distractions as easily as a Three will, but, since they have more sensitive hearing, sounds may still throw them off.
Assessment: Threat; avoid. If fighting is necessary, aim for the head.
Class 1: Ones/Firsts
Ones are the rarest, yet most dangerous class of zombie. They are instantly recognizable from their ghost white skin and pure, pitch black eyes. A One will carry itself like a normal, conscious human would— it'll walk with its back straight and lift its feet off of the ground instead of dragging them as a Three would do. Ones do not let out snarls, groans, or anything like that— they usually don't vocalize so that they can stay hidden from any survivors that might be around. Usually, a One will recognize the presence of a human faster than the human realizes that the One is there, and by then, it's too late. When they are recognized, though, a First can and will speak to people. They can form complex, coherent sentences, and even have the ability to mimic voices that they've heard instead of being restricted to solely their own voice.
Ones are extremely aggressive, but also know when to retreat in the rare instance that they get hurt. Their intelligence levels are even higher than that of a Two's— in fact, oftentimes, they're smarter than survivors. They can do anything from lock picking a locked door to spying on people until the right time to go in for the kill comes around. They can register faces, names, and important information said by survivors in their head, and will remember everything. They know how to read, write, and if it comes down to it, will negotiate or take people hostage. Due to their advanced speech and mimicking abilities, they will imitate the voice of another person in order to lure a close friend or companion that they were with in.
Other zombies seem to have a sense of fear, admiration, and respect for Ones. In fact, I've heard rumors that they've been seen leading and commanding massive hordes of lower zombies, taking strength in numbers to a whole new level. Ones will pursue a target until they are dead, but if they happen to lose a potential meal, will attempt to figure out where they've gone and chase after them.
Ones can have 2-5x the amount of physical strength and speed that a Two has, making them almost impossible to outrun or beat in hand-to-hand combat. They have incredibly tough skin and thick armor plates protecting most of their vital areas, and even if they didn't, they have such coordinated, precise attack patterns that they are extraordinarily difficult to land a solid hit on. They even have better senses than other zombies— in fact, they can even see better and will amble about during the day. If you have to engage in combat with a One, then aim for the head and only for the head. Destruction or damage to the brain will result in death. Don't bother distracting these things with anything but light— something like a flashbang or explosive will temporarily blind them and cause disorientation. They are too smart to be distracted by sound, even if their ears are sensitive.
Assessment: Extreme threat; avoid at all costs. If they spot you or you suspect one is in the area, run. Do not engage in combat if you can avoid it.
Unknowns/Anomalies
These are zombies that don't meet the appearance, abilities, or standards of any of the previously mentioned classes. Their abilities and behavior vary— they can be intelligent or dumb as a rock. Strong or weak— in fact, I've heard that some can be more powerful than Ones or even weaker than a Three. Maybe there could even be a friendly one out there with how weird these things are. There isn't much information on this species of infected— just that they're unpredictable and quite scarce.
Assessment: Unknown. More information needed; approach with caution.
Winter 
Day 
Sleet
