What Is Disease? Definition, Causes, Types & Prevention

Disease is any deviation from normal physiological structure or function that impairs homeostasis. It differs from illness (subjective experience) and sickness (social role).

Etiologic Categories

• Genetic: Mutations (inherited or de novo) → cystic fibrosis, sickle cell, Huntington's
• Environmental: Toxins, radiation, pathogens, allergens
• Lifestyle: Tobacco, diet, inactivity, alcohol, sleep deprivation
• Aging: Cumulative molecular damage, immunosenescence, neurodegeneration
• Multifactorial: Most chronic diseases (CAD, T2DM, cancer) involve gene-environment interaction

Major Disease Burdens (WHO)

• Cardiovascular: Ischemic heart disease, stroke — leading global cause of death
• Neoplasms: Uncontrolled cell growth; lung, breast, colorectal, prostate most common
• Diabetes: Insulin deficiency (T1) or resistance (T2) → hyperglycemia, micro/macrovascular complications
• Chronic respiratory: COPD, asthma — airflow limitation
• Mental/neurological: Depression, anxiety, dementia, substance use

Prevention & Control

• Primordial/primary: Vaccination, sanitation, tobacco control, healthy diet, exercise
• Secondary: Screening (mammography, colonoscopy, BP, lipids, HbA1c)
• Tertiary: Disease management, rehabilitation, palliative care

Health equity, access to care, and social determinants critically influence outcomes.