Wiping off the sweat from his biceps, Jack feels his fingers slide through a soft layering of blondish hair. "What the hell's going on?" he curses, lifting up off his shirt, his toned, firm six-pack caked in newly grown light, velvety fur.
Tossing his shirt aside, the hair becoming ever thicker, he watches nervously as the skin beneath it starts to take on a richer, darker tan, its baby-smooth feel changing into a hide-like, rougher texture.
"Carlos, dude..." Jack says with detectable fear in his voice, "I need to go see the medic, something's wrong with me, man..."
Carlos, busily massaging his groin as he ogles the Playboy's centerfold, dismissively remarks without even a glance at Jack, "OK, man, go ahead..."
Suddenly, Jack shivers as he unexpectedly feels a breeze of cool, open air on his manhood. And, looking down, his jaw drops as he notices his cock has grown over his pant's zipper, its new weight pulling the zipper down. A long, mottled, fleshy tube sticks up and out from his hairy crotch, rows of brown, furry skin surrounding its wide base in the beginnings of a sheath. His glans look distinctively different too, flatter than normal, his dick grotesquely thick and lined with veins, its color rapidly turning ebony, interspersed with pink patches near the flared head.
Ears sharp and pointy, Jack cringes and falls to his knees as he feels his legs begin to re-arrange, his hamstrings and quadriceps thickening, threatening to burst from his tight pants, as his body consumes what little fat he had to replace it with striated, lean muscle. Toes curling up, he stretches around to watch in horror as his boots fall from his elongating feet, his toenails fusing and darkening into hooves.
"Carlos!" Jack shouts, almost whinnies, his vision changing as he feels his eyes move off to his face's sides, pushed apart by a growing muzzle, his back on fire as hair sprouts rapidly there into a voluptuous, long, flowing mane.
Carlos finally lowers the Playboy, his hand quickly drawn away from his dick as he sees the freakish half-man, half-horse Jack changing before him: it still possessing predominantly recognizable features of his buddy. "Holy shit, man!." he screams, unaware that his own body has also begun the changes into stallion-man from the infected canteen he drank off.
Jack's pants, the only article of clothing not shredded from his alterations (yes, he wasn't wearing underwear), groans, on the verge of completely splitting and giving way to leave his transforming body completely naked to his bunkmate.