Understanding the Causes of Disease: A Comprehensive Framework

Disease arises from complex interactions between biological, environmental, and behavioral factors. Modern medicine categorizes causes into broad domains that often overlap.

1. Infectious Agents (Pathogens)

• Bacteria: Tuberculosis, strep throat, UTIs, pneumonia
• Viruses: Influenza, HIV, HPV, hepatitis, COVID-19
• Fungi: Candidiasis, aspergillosis, ringworm
• Parasites: Malaria, giardiasis, toxoplasmosis
• Prions: Creutzfeldt-Jakob disease

2. Genetic & Congenital Factors

• Single-gene mutations: Cystic fibrosis, sickle cell disease, Huntington's
• Chromosomal abnormalities: Down syndrome, Turner syndrome
• Multifactorial inheritance: Heart disease, diabetes, cleft lip (genes + environment)
• Congenital anomalies: Structural defects present at birth (e.g., heart defects, neural tube defects)

3. Environmental Exposures

• Physical: Ionizing radiation (cancer), UV radiation (skin cancer), noise (hearing loss)
• Chemical: Air pollution (asthma, lung cancer), asbestos (mesothelioma), lead (neurotoxicity), tobacco smoke (COPD, cancer), occupational carcinogens
• Biological: Mold (allergies, asthma), vector-borne diseases (Lyme, dengue)
• Social/Built Environment: Food deserts, lack of green space, unsafe housing, poverty

4. Lifestyle & Behavioral Factors

• Diet: High ultra-processed food, sugar, saturated fat, sodium → obesity, CVD, diabetes, cancer
• Physical Inactivity: Independent risk factor for CVD, diabetes, osteoporosis, depression
• Substance Use: Tobacco (cancer, CVD, COPD), alcohol (liver disease, cancer, injury), illicit drugs
• Sleep Deprivation: Metabolic dysregulation, immune suppression, cognitive decline
• Chronic Stress: HPA axis dysregulation → inflammation, CVD, mental illness

5. Immune System Dysfunction

• Primary Immunodeficiency: Genetic defects in immune components (e.g., SCID, CVID)
• Secondary Immunodeficiency: HIV, malnutrition, chemotherapy, corticosteroids, diabetes, aging
• Autoimmunity: Immune system attacks self (RA, lupus, type 1 diabetes, MS)
• Allergy/Hypersensitivity: Anaphylaxis, asthma, atopic dermatitis

6. Metabolic & Degenerative Processes

• Metabolic Syndrome: Insulin resistance, dyslipidemia, hypertension, central obesity
• Neurodegeneration: Alzheimer's, Parkinson's (protein misfolding, neuronal loss)
• Organ Failure: Heart, kidney, liver, lung — end-stage of chronic diseases

Major Disease Categories & Key Drivers

• Cardiovascular Disease: Hypertension, dyslipidemia, smoking, diabetes, obesity, inactivity, genetics
• Stroke: Hypertension, atrial fibrillation, smoking, diabetes, carotid stenosis
• Cancer: Tobacco, diet, infections (HPV, HBV, HCV, H. pylori), radiation, chemicals, genetics, aging
• Chronic Respiratory Disease: Smoking, air pollution, occupational dusts/chemicals, allergens, genetics
• Diabetes (Type 2): Obesity, inactivity, genetics, aging, gestational diabetes history, PCOS

The "Web of Causation" Model

Most diseases result from multiple interacting factors across these domains. For example, type 2 diabetes involves genetic susceptibility + obesogenic environment + sedentary lifestyle + poor diet + epigenetic changes + microbiome alterations + chronic inflammation. This framework guides prevention: targeting modifiable factors at individual and population levels yields the greatest impact.