Dib had started to notice that Zim's body had started to change. First he started to grow taller and a bit bulky. Then he started having strange cravings and his nails started to look more like claws.
"Hey Dib." Zim said nervously.
"Yeah?" Dib asked.
"I found out what's going on with my body." He answered.
"Well, is it serious?' Dib asked worried.
"It turns out that I have a rare Irken disease. The good news is it's not fatal but the bad news is I don't think I'll be the same. The disease is making my body evolve backwards. About a million years a day." Zim answered.
"Is there a cure?" Dib asked.
"Yes but it takes a year to make." Zim said.
Four months later:
Zim was now seven feet tall, had a full head of hair, one inch long tusks, a few tuffs of fur, claws, and a pot belly.
"Does Dib still love Zim?" He asked.
"Of course, I'll always love you!" Dib answered as he gently stroked the large Irken's furry chest being rewarded by a gentle nuzzle in the crook of his neck.