The year is 201X. A new pathogen is discovered in the wreckage of a ship buried in the arctic ice. After it infects the entire research team resulting in their quarantine, experiments are performed in a lab deep in the mountains of the United States. The way this pseudo-virus operates is outside the bounds of anything this world has ever seen before.
Instead of killing or harming its hosts, it changes them into a higher evolved version of themselves. They become stronger, faster, smarter, but all of these are devoted to one thing and one thing only: the spread.
All hosts experience 4 stages of infection, with the 4th being the final, and permanent stage of their transformation. Transformation, depending on how they are infected can range anywhere from 24 hours, to 15 minutes.

Document recovered from Site [REDACTED] after a full containment breach. Doctor Ross, the author of this document, is currently MIA, assumed infected.

"Stage 1: Depending on the infection vector, hosts experience flu-like symptoms for a period of 24 hours. Though, if directly infected through bodily fluids (eg: semen, saliva, blood, etc.) subjects skip Stage 1 entirely.
Stage 2: Subjects begin to experience the first stages of their transformation. In men and those with male sexual organs, muscle mass, height, and testosterone levels increase. In women and those with female sexual organs, they experience similar symptoms with a decrease in breast volume over time, and the beginning of penis growth.
Stage 3: While subjects continue their physical change, mental changes begin. Subjects start to become enamoured by men, specifically those infected with the G-4Y pathogen. Those who were infected via water or air might begin searching out active infected to engage in intercourse with. At the end of this stage, subjects of all genders and body types average around 6'1", though exceptions are common. At the end of this stage mentally, infected have begun to view the virus as a "gift" and they repeatedly express interest in "sharing".
Stage 4: The fourth and final stage results in drastic bodily growth. All hosts, now fully biologically male with almost no deviations from this norm, average out with a height of 6'5", and weight of 230Lbs (mostly muscle mass). Penis averages out at 15 inches, with deviation only being 3 inches above or below in most recorded cases. Testicles have been repurposed, and semen is no longer fertile, only serving as the primary infection vector for the G-4Y pathogen. In 100% of cases, infected will actively seek out healthy hosts to force into sexual intercourse, usually using their powerful pheromone rich musk to overpower their thought process and send them into Stage 2 before intercourse begins.
It is the recommendation of this researcher that..."

The document cuts off here, and it is shown via onsite security cameras that Doctor Ross is dragged off-screen by infected individuals. No one returns to the office, and hours later, the site loses power.