Short answers, pulled from the story.
A disease is a particular abnormal condition that adversely affects the structure or function of all or part of an organism and is not immediately due to any external injury. In humans, the term is often used more broadly for any condition that causes pain, dysfunction, distress, social problems, or death.
The four main types of disease are infectious diseases, deficiency diseases, hereditary diseases, and physiological diseases. Diseases can also be classified in other ways, such as communicable versus non-communicable.
The deadliest disease in humans is ischemic heart disease, an obstruction of blood flow, followed by stroke and COPD, a lung disease. In developed countries, neuropsychiatric conditions such as depression and anxiety cause the most sickness overall.
A primary disease is due to a root cause of illness, while a secondary disease is a sequela or complication caused by the primary disease. For example, a common cold is a primary disease and the rhinitis that follows can be a secondary disease.
Disease burden is measured by financial cost, mortality, morbidity, and indicators like years of potential life lost (YPLL) and disability-adjusted life years (DALY). In 2004, the World Health Organization calculated that 932 million years of potential life and 1.5 billion disability-adjusted life years were lost to disease and injury.
The most known and used classification of diseases is the World Health Organization's ICD, which is periodically updated. The most recent publication is the ICD-11.