Slime mold cell structure
The slime mold is made by a lot of multinucleate amoeba-like cells. They are multinucleate in order to get better surviving chances if a cell is chopped in two halves. But they aren't very dense in the number of nucleus to avoid consuming too much energy.
The cells have a lot of signaling mechanisms, like neurons do and also are capable of differentiating into different specialized cell types.
Also, the cells have some chloroplasts in order to get some energy from the sun and melanin in order to get into the sun and be protected from UV rays.
Further, the slime cell also features an immune system similar to the one existing in plants, and the chloroplasts are important for that.
In fact, it is not a simple slime mold. Since they are genetic engineered, they are very complex transgenic slime molds organisms with genes coming from fungi, animals, plants, bacteria and some novel ones created specifically for it.
Infection level 1: Breathing skin
The slime mold would also need to breath oxygen. If it doesn't have access to oxygen, it would have to have a very slow anaerobic metabolism and it would make the zombie unable to walk and also unable to self-mummify quickly enough to fight off body decay and co-infections. So, once established, the first thing that the slime mold would need is to get access to oxygen. Also, since it lacks a circulatory system, the slime mold would need to diffuse oxygen from cell to cell.
The best way to get enough oxygen is to wrap around all the skin with specialized respiratory cells. Also, since the skin or an open sore is the most probably place where the infection starts, this might start right from the start. So, to simplify the biochemistry, let's assume that the slime mold is simply unable to grow without oxygen.
Once established on the skin of the victim or into an open sore, the slime mold reproduces very fast in its skin border areas in border in order to conquer all the skin. To get enough energy for that, it would also need to consume some flesh in those border areas. However, since it also needs the most preserved flesh, it eats just the minimum necessary.
Further, in order to optimize its energy and oxygen needs, those skin cells are large in area but thin in thickness. Also, they are actually double layered: One layer is over the skin and another one under the skin. Between those cell layers there is the host dead skin or perhaps some other exposed host tissue.
Both cell layers are purposed in conquering the skin. The outer layer also have the purpose of getting oxygen, giving oxygen to its neighboring slime cells, exporting waste dissolved but concentrated in water (like sweat or urine). The inner layer is purposed to regrow the outer layer if it is removed (e.g. by washing, showering or abrasion).
Further, both cell layers communicate in a way that if the outer cell layer is intoxicated or damaged (e.g. by soap or gelified alcohol), they break their connection until the outer layer gets better and restarts orderly or the inner layer regrow an outer one. The connections are small synaptic-like or hypha-like threads between the cells. Since those cells are large in area, they don't need too much of those threads to communicate, and this also avoids overcrowding the host skin with those threads.
Since breathing would consume oxygen and produce carbon dioxide, this means that breathing loses carbon. They solve this by having chloroplasts, get sunbathing and doing photosynthesis. The photosynthesis absorbs almost all of the carbon dioxide, at least during the day, it also consumes some carbon dioxide from the atmosphere. Working chloroplasts are multiplied in the outer skin areas only when they get sunlight. Slime mold cells that don't get sunlight keep just a few inactive chloroplasts.
Also, the slime mold drinks water when the zombie enters a river, a pool or a sewage. A zombie kept with no access of water would die from thirsty.
The result is that the zombie breaths like an hybrid of a plant and a amphibian. Also, they have greenish skin. Also, they have foul smell since their excretion is all through the skin.
Finally, when conquering skin area, the slime mold would eventually enter the mouth, the nostrils, the ears, the eyes and the anus. By conquering the lungs surface, it gets a lot of breathing area.
Infection level 2: Flesh zombification
Once a patch of skin is conquered, the slime molds far from its borders enters its second level of infection: Mummify the area.
The slime mold produces substances to preserve the body and fight off co-infections of competing bacteria and fungi and also slow down the body decay. I.E. a zombie is self-mummifying. A way to make that happen is to produce a lot of nasty toxic chemicals that only the slime mold itself is immune to. This also have the side effect of poisoning and killing its host if it happens to still be a living person. Further, this also works as an immune system for the zombie.
Zombie-skin cells in this state reproduces much more slowly in order to avoid consuming the body too quickly or getting depleted from water, oxygen or energy. The outer layer just mostly supply oxygen and photosynthesized food for the rest of the slime mold. But the inner layer detects if there is non-mummified tissue and produce mummifying toxins that they release into those tissues. Further, they get the raw chemical materials for fabricating those toxins from the very non-mummified tissue that they are mummifying.
Note that a patch of infected skin would have borders in the first stage and its center in the second stage (or further stages). It isn't all in one stage. Each cell has its own stage and they look into their neighborhood and communicating with their neighboring slime-mold cells in order to determine their own stage and function.
Infection level 3: Getting to bones
Once a skin cell has no nearby unconquered skin and all the nearby flesh is mummified, it enters it third stage: Look for the bones. The slime mold inner skin layer cells will project hypha-like threads down away from the skin. When they find a bone, they create a specialized bone-bordering cell that have the purpose of controlling the skeleton. Also, those hypha-like threads will create ramifications in order to mummify the flesh. They also supply oxygen down into the bone-bordering cells.
Once the bone-bordering cells forms, they start to spread in order to conquer the bone surface just as the outer cells did.
The bone-bordering cells signals each other with the purpose of measuring the bone if the bone is completely conquered or not (bone-bordering cells that feels a unconquered area answer a loud chemical NO to that signal and the other cells around the same bone spread out that NO). Once the NO signal don't get answered
Infection level 4: Connecting the bones
Bone-bordering cell without nearby unconquered bone-border area nor without neighboring hypha-like threads from skin-layer cells will try to find other bone-bordering cells that aren't immediate neighbors. They do so by projecting out hypha like thread until they find another bone-bordering cell. Once two bone bordering cells contact, they will try to pull back and shorten that thread but not with enough force to break it. If necessary, the thread anchoring can slide under the cell surface in order to find the best anchoring place. This also have the purpose to make the connecting thread as short as possible. If the thread ends being a very short almost-zero-length bridge from two neighboring cells, it is deemed as unnecessary and broken down.
Those bone-bordering cells try to contact nearby bones quickly when they are newly formed and their conditions for doing so happens. But once they are long enough in that state, they quiet down and only for once in a while they tries to connect to another bone. Instead those old enough cells focus most in doing the next task: infection level 5.
Infection level 5: Making out slime-mold muscles
Muscles are necessary to have force. So, the bone-bordering cells that completed their level 4 tasks and couldn't connect to other bones starts to consume flesh in order to create muscle-like cells around them and those newly formed muscle-like cells do the same until they are signaled from the slime-mold skin cells that they are too close.
Infection level 6: Making out slime-mold nerves
Muscle-like cells signals each other in order to elect which one would turn into nerves-like cells. Locally, they can survey the neighboring cells in order to determine in which plane the bone-bordering cells and the skin-cells are, and the cells elected to be nerves should be in a layer in the middle of these.
Surveys are done by sending cell-signals into nearby cells, amplifying those signals into other nearby cells and comparing signals received from one direction to signals received from other directions. With this, cells can roughly determine which ones should be turned into nerves and in which direction the nerves should run.
With those surveys, the cells should be able to roughly measure the bones, and then, those nerve-like cells will try to connect with each other in the direction that the bones are the longest. Further, once the nerve-like threads forms, they will try to rectify themselves with both cell-migrations and re-elections. Nerve-like-cells that are deemed unnecessary revert back to muscle-like-cells. Since those surveys occur in the muscle-like-layer only very-roughly knowing where the bones and the skin, it is able to jump over different bones and eventually connect the entire body.
The cell-surveys also have the purpose to make nerves lose ends join together to reconnect or perhaps determine that they are the actual nerve endings in the tip of the fingers or toes.
Y-shaped junctions will also progressively separate into disjoint threads. In order to know in which direction they should separate (i.e. a Y-shaped connection in the hand connecting two fingers and the forearm should separate in the direction of a forearm, not in the direction of one of the fingers), they survey for knowing in which direction is the nerve-endings in order to disjoint them into the opposite direction.
With this, nerves will eventually a net. They will connect the toes and other loose endings in the feet and legs through the spine in the direction of the head. Endings in the torso will also be guided by the ribs to find their way to the head. Nerves in the arms will connect the fingers and other loose endings in the hands and arms through the shoulder and neck into the head.
Infection level 7: BRAINS!
Zombies don't need brains to live, but those that have brains are much tougher and better than those that doesn't. When the slime mold gets into the brain, it tries to conquer it as quickly as possible.
Neurons have structure and that structure will decay quickly if nothing is done to preserve them. And here, the slime mold try to attack the neurons and imitate their structure as quickly as possible before it decays.
Once the slime mold can possess the brain, mummify it, eat it and imitate its former structure, we will have a zombie that have some memories, some knowledge, some abilities, some intelligence and possibly even be capable of speaking. Also, since the slime-molded brain isn't immutable, it also even have some learning capability! Of course, since the brain-conquering process is inherently imperfect and must be done very quickly in order to work, brained zombies are still stupid and have only a fraction and the behavior of their former selves. Also, the imitation brain works much slower than a true brain. But this might be enough to:
Have a zombie walk in a house chasing victims without stumbling over walls.
Have a zombie getting a street cone and using it as a helmet.
Have a zombie using the elevator and knowing to which floor it wants to go.
Have a zombie getting a ladder in order to jump over a wall.
Have a zombie with a crowbar smashing a window glass and a locked handle in order to enter a house.
Have a zombie posing as a beggar in order to approach victims without making them notice that until it is too late.
Have a zombie saying "Breeeiiinzzzz, Iii wwwaaaant breeeeiiiinzzzz!"
Have another zombie saying "Ssstteeeeeveee, uuu'lll jooooinn uuuzz nnooooowww!"
Have a zombie writing a letter to a victim "Hello, This iz your Muther. Please come over to myhouse for "Meatloaf" Leave your front door open and your lawn unguarded. Sincerely, Mom (not the Zombies)"
Have a zombie driving a car without crashing.
Have a zombie grabbing a gun and then saying "IIITTSSZZ PPLLAAAYYYTTIIIMMEE! HAA HAAA HAAAA!"
Have a zombie writing a love letter "Carol, come back, please! We lived together and now we can unlive together!"
Have a zombie opening its e-mail and looking for how to reach its next victim on facebook.
Have a zombie ordering stuff from Amazon and paying with a credit card. When the intended package arrives, it also tries to kill the deliveryman and rob the truck.
Have a zombie telling an human: "Wweee aaarrr ddeee zzooombiiis. Uuu'lll beee aaziimmmiillaateeed. Reeezziizzttaansee iiizz ffuutiillee!"
Have a zombie telling his wife: "Liillyyy, Iii loooovveeee uuuu! Kiilll mmee anndd rrruunnn! Ssaaveee uur ssooonnnn!"
Infection level 8: Expiration
Eventually, the slime mold will need to eat all of the flesh and will starve. The zombie could still try to find something to eat, possibly an animal, and by being able to eat, it can feed the slime mold. Vegetable food is also possible, but without a working digestive system, that type of food is very hard to consume for the slime mold, so they always prefer flesh. Also since even the best zombies have poor coordination, poor intelligence and are very improbable to get sufficiently fed by living humans, most will eventually starve.
Fat zombies tend to unlive more than thin zombies, exactly because the mold will get fed on dead fat.
Also, although they can eat and digest food in their stomach and intestines, they might also eat by just attaching the food somewhere in the skin and waiting the slime mold do its job.
Finally, zombies very much prefer to attack living victims in the neck or in the head in order to make the slime mold gets into the brain as quickly as possible and conquer it with the minimal possible decay. It even evolves and gets natural selection for that.
And animals?
If the slime mold conquers the body of a dog, guess what? It becomes a zombie dog! A cat body? A zombie cat!
And also zombie fish, zombie rabbits, zombie cows, zombie horses, zombie snakes, zombie frogs, zombie elephants, zombie turtles, zombie mice, zombie lions, zombie bears, zombie tigers, zombie giraffes, zombie sharks, zombie monkeys, zombie whales...
However, in order to make the zombie animals breath efficiently, they would shed most of their fur.
We could also have zombie birds, but due to their poor coordination and the shedding of their feathers, they would be unable to fly.
Zombie insects probably won't happen because they are too small and their exoskeleton don't play nicely with the breathing requirements of the zombies.
Zombie plants also won't happen because it is hard to be digested.
Damaged zombies: How they recover?
Bone-bordering cells that are frequently disturbed and injured will eventually decide that it is enough and will turn into a hard wood-like substance. Also, those wood-like cells glue themselves into the neighboring bones and other neighboring wood-like cells. This make broken bones eventually reconnect.
Damaged skin just needs to be reconquered. Damaged flesh will grow new hypha-like cells. Damaged muscle will regrow. Cut-down nerves will reconnect since nerve cells surveys the nearby tissue with the very purpose of forming, reforming and reconnecting.
If the slime mold don't find a brain, it will make an improvised one somewhere in the body where the nerves converge. The improvised brain is poor, but the slime mold have the needed structure encoded in its DNA. It is enough to form a brain-imitation with the complexity of a fish-like, mouse-like or bird-like brain, which is enough to give it an animal-like intelligence and behavior sufficient for walking, finding food and water, avoid enemies and dangers, search for victims to infect and have a minimal intelligence and memory in order to survive. But it won't be able to talk, drive vehicles, operate machines or use a computer.
So, if you cut out the head of a zombie, the zombie head continues to try to behave as a zombie, but it would be unable to do much and would run out of oxygen very soon. The body will fall down and the slime mold would start to construct a new brain somewhere in the torso. When the brain is powerful enough, so you might see headless zombies walking around.
But the zombie head would not die quickly or easily. Since the flesh is mummified, it well-preserved. When the slime mold runs out of oxygen, it goes into a dormant very-low-metabolism state instead of just dying. So, the zombie head would not decay or will do so very slowly and it still can be reanimated. Also, a zombie head, although not having enough oxygen for complex thought, still has enough for a minimal metabolism.
Also, it have the ability to form eyes. It may just mummify and zombify its host former eyes. But if the eyes are missing or in a very bad shape, the slime mold knows how to grow new eyes and know where they should be. A blind person that is zombified becomes a sighted zombie!
Self-frankensteining!
A headless zombie will grow eyes somewhere in the skin where it receives sunlight and is far from bone-joints. So headless zombies might have eyes in their arms, in their necks, in their backs, in their chests, in their bellies, in their butts, in their legs... And also, they can have a lot of eyes.
Further, zombies parts might rejoin. If a headless zombie finds out a head, it might get that head and hold it firmly over its own neck for some hours or even a few days until the slime molds in both parts join and create muscles, skin, nerves and connecting wooden-bones and make it firm enough to not break easily. It doesn't even needs to be its former head, it could be the head of some other person or even the head of some animal.
If you cut out an arm, the slime mold will try to create a small brain in that arm and you might see an eyed-arm crawling around by moving its fingers. It will also perceive itself as too small to be functional and with try to find a more functional zombie somewhere to join into it.
Also, in order to improve the joining process, it can grab some materials like wood planks, sticks, bamboo, iron bars, duct tape, bandage, water tubes or whatever that it can use to hold the two bodily pieces together more firmly. It may also fill voids and holes with beef, raw hamburgers, fish-fillets, rat-flesh or anything like that.
An incapacitated zombie with what would be a fatal wound for its host is capable of re-raising after some time. This is the time the slime mold takes to reconnect and repair its tissues and remummify the flesh if needed. However, it might need to consume some flesh in order to do that, so a very badly damaged zombie might be incapable of re-raising again. Also, a zombie can't be re-raised an indefinite number of times if it does not eat, find filling flesh or join with other zombie parts in the mean time.
Also, some zombies may turn out into very horrendous monsters by joining parts of different bodies:
A Goro-like four-armed zombie monster is possible!
Two-headed zombies? No problem!
What about three-headed or four-headed zombies?
What about a centaur zombie, half human-zombie and half horse-zombie?
A zombie human centipede?
What about a monster with 8 heads, 3 torsos, 27 eyes, 5 butts, 13 arms and 9 legs, with not all of that being human parts, and it fights with swords, axes, hammers and guns all at the same time?
What about a zombie that after having its brain taken over by the slime mold, have the slime mold grow and improve the brain until its become an ultraintelligent zombie?