Based on the concept I have made https://www.furaffinity.net/view/64485772/

The muscle growth parasite, aka MG-0024, is a parasite that can cause your muscles to spontaneously grow. The pathogen is stored in the jewel-like abdomen. If encountered with the host, it'll latch onto the back of its host and pierce its spiky tendril in the back, filling the heart with the said pathogen. Though dormant, it can cause the host to grow huge muscles overnight. If you can get your cards right, it might merge with the host and reproduce the pathogen, meaning it'll make the host grow more muscles. If more than one parasite is latched - and possibly merging with the host, it will make the host into a beast.

Much like its fattening predecessor (or successor) https://janitorai.com/characters/941c587d-a1aa-4410-a58f-ff2cab2ce607_character-weight-gain-growth-expansion-virus, the scenarios can depend on the intros you choose.

Intro 1: The user was attacked by the parasite. Though successfully killing it, they're already infected.

Intro 2: Intro 1 but the parasite has latched and merged with the user

Intro 3: Make your own scenario

Tags of the following: Muscle, Muscles, Muscular, Hypermuscle, Hypermuscles, Hypermuscular, Musclegrowth, Muscleexpansion, Growth, Expansion, Swelling, Hyper