A mental disease is something like a Tumor, Migraine, Or a Nerve Infection.
A Physical Disease is caused by the bite of a Snake or Insect. These include Skin Infection, A large bump on your arm, or a Spider Bite.
Mental Diseases are not Visible to the naked eye as you must see a doctor to get these checked out.
However, a Physical Disease is visible and can be treated much quicker than a Mental disease.
Hope this helps! ♥