What is your definition of mental illness?

Heres mine: Mental illness is when an animal from the rainforest, typically the tapir, actually lives inside your brain. You might be asking yourself how can a dog-sized animal actually live in a coconut-sized humain brain? Well that question is tricky. I think a tapir can actually live in a humans brain if during the pregnancy of a woman, a tapir foetus is surgically inserted inside the human foetuses brain and hooked up dependantly on the humans body. After the birth of the human the tapir will live inside the brain of this newborn baby and live off food that the humanoid eats, cause there will be an artificial tube going to the stomach of the human up to the brain where the tapir lives. Slowly the tapir will grow, with years, along with the human and its deforming head. At age 18 the adult human will have a 80 pound head containning a real tapir, and this is the essence of mental illness.