How could a wolf-man be explained without magic?

Question

The legends of lycanthropes go back eons; while they have changed over time, a common modern expression of them is of an infected or cursed human that is given the power to transform into a wolf and/or a hybrid canid-humanoid form either willingly or when triggered by certain events (anger, full moon), in exchange for a more...primal and predatory mindset. While we know that shapeshifting requires magic, one possible interpretation of lycanthropy is of a permanent change to the "hybrid" canid-humanoid form upon infection:

• Thicker hair/fur, often a countershaded grey although other colors are acceptable (black is common in the mythos, but is the result of introgression with domestic dogs in real-life wolf populations)
• Canid head structure (muzzle + strong bite, dentition, set-back eyes, erect ears) and largely canid senses (esp. hearing and smell, but with good eyesight too, while retaining human trichromacy)
• Fixed claws instead of nails on both hands and feet
• Digitigrade, bipedal locomotion
• Growth spurt upon contraction (both height and weight/strength)
• Athletic and strong (easily capable of wrestling an average human to the ground or pinning them to a tree, and running them down for that matter)
• Adaptable, largely carnivorous diet
• Capable of eating large portions in one sitting
• Can pass these changes along to uninfected humans with a bite, or to their children
• Human intelligence in spite of cranial capacity restrictions, with only modest personality changes (i.e. they aren't totally feral/could learn to function in human society)
• Fully fertile with other lycanthropes (werewolf puppies \o/)

(How) would this one-time transformation be possible biologically (i.e. explainable in terms of what we know about how biology works, even if it'd be difficult/improbable/require bioengineering to create)?

Answer 1

There are organisms called Retro Viruses that are capable of altering host cell DNA causing mutations.

Some viruses mutate DNA so much it causes cancer the runaway/unregulated growth of cells.

Indeed viruses have been looked at heavily as a means of genetic engineering existing organisms. Obviously it is met with extreme levels of controversy like what happens if it doesn't work right or enters a completely different organism or becomes transmittable. Very scary stuff.

Engineering a lycanthropy virus would be very difficult:

• Most viruses are designed to attack certain cell types, for lycanthropy you would need to target just about all, bit of a stretch. This is particularly difficult with brain, because it is one of the most protected organs in the body. Not impossible with achievers like meningitis.
• A virus is good because it does fit the transmissible attributes: bites, however it would be transmissible through just about any other fluid transfer avenue except maybe urine. This does include sexually transmitted lycanthropy.
• The hard part for making lycanthropy hereditary is, if you have a virus capable of altering every cell in the body it would also alter reproductive cells. This is bad because if these cells don't match up with the partner a viable offspring might not be achievable. So the virus would need to not mess with these cells and instead just lay dormant when being passed through reproductive relations. HIV has had instances of such dormancy.
• Bones are difficult. Bones aren't alive but are constructed by cells. An interesting thing about bones is that cells can also eat them. Particularly in the elderly cells can take back minerals from the bones degrading them over time. If you consider it takes roughly 20 years for a baby to make an adult sized bone this is not a quick process.
• teeth, similar to bones, are another hard construct that are very difficult to produce taking roughly 8-9 years for adult teeth to erupt.

Time is the real key to making this realistic

20 years for skeletal changes and 8 years for oral reconstruction is not feasible for a werewolf. Fortunately the body measures time in a process called metabolism. Metabolism is the activity of cells and if your virus can speed up this process you can affect changes more rapidly.

The advantages in increasing metabolism is you increase demand for food energy (hunger) with meat being a very good source for most of the bodies required nutrients. Other required nutrients can be acquired through other parts of animal carcasses: eyes, liver, bones, etc. Another advantage is you increase regeneration making many wounds quickly repairable (another aspect of lycanthropy).

Progression:

Invading all the cells in the body would take weeks with metabolic rate slowly increasing. Cells closer to the blood stream would be affected first like muscles and organs. Changes would be slow like increased athleticism and food intake. As more cells become infected food demand would increase meaning more hunger, more demand on nutrient dense sources (meat). Hunger alone triggers primal instincts in the brain like aggression. Eventually more distant cells like nerves and hair follicles will become enhanced, likely rewiring the brain for enhanced aggression and processing of smell, hearing, and vision. After possibly months of that you would begin to see the restructuring of hard parts. How long that would take exactly is unknowable, but it would be months in the least and be very painful.

Answer 2

Humans don't change like insects. We don't have genes for this. An you need genes if you want to pass changes to puppies. So, you need a genetic alteration, and this is possible via a virus infection, what match with your form of contamination.

Normally virus only change the genes of the cells to make more viruses, and not to do macro scale body transformation. so, a artificial virus is highly probable.

This is the ground. The details are a lot more hard. skeletal changes are hard. Bones are a mineral porous structure with live cells inside them. Dissolving the structure to allow change in the shape of the body and skull is a highly complex matter.

But the hardest is the fertility. Fertility implies genetic compatibility, which means that the infected homo sapiens turn to another species after this process. So, the infection have to be mostly reproducible and controlled, AND do the SAME changes they do to tissues cells to gametes. The most complex problem is that viruses are normally bound to some type of cell. AIDS to lymphocytes, Ebola to phagocytes, and so.

There is none example in nature to a virus capable of such complex transformation and to pass the changes to descendants of the infected.

The only other way I can see is nanorobots, that change the DNA of the infected person.