Short answers, pulled from the story.
A disease is an abnormal condition that negatively affects the structure or function of an organism without immediate external injury. This definition separates true pathology from temporary discomfort or injury.
Four main types of human disease exist: infectious diseases, deficiency diseases, hereditary diseases, and physiological diseases. Some classifications rely on cause while others focus on pathogenesis or symptoms.
In 2004 the World Health Organization calculated that 932 million years of potential life were lost to premature death globally. That same year they determined 1.5 billion disability-adjusted life years were lost to disease and injury worldwide.
Obesity was once a status symbol in Renaissance culture as seen in Andrea Sacchi's painting The Tuscan General Alessandro del Borro from 1645. Today it is generally regarded as a disease despite historical perceptions persisting in many African regions especially since the beginning of the HIV/AIDS epidemic.
The term illness refers specifically to the patient's personal, subjective experience of their condition. Sickness behavior includes evolved responses like lethargy, depression, loss of appetite, sleepiness, hyperalgesia, and inability to concentrate.