This is troubling. If you didn't use a nuclear attack how could the trashipitus virus have mutated? Could the virus have mutated from something other than the artillery attack? We need to think of everything that interacted with AHHHHHHHHHHHHHHHH between the destruction of titan AHHHHHHHHHHHHHHHHHHHHHH and is mutation that allows him to multiply himself.
I heard Royal scientist Abraker took him into his laboratory right after AHHHHHHHHHHHHHH came back to normal, he most likely interacted with something in that laboratory, with or without the knowledge of the royal scientist.