A practical framework for understanding infectious and noninfectious disease
At a Glance
Question | Key idea |
|---|---|
What is disease? | A disturbance of normal structure or function that produces characteristic signs, symptoms, or measurable abnormalities. |
Infectious? | Caused by a transmissible biological agent such as a virus, bacterium, fungus, parasite, or prion. |
Noninfectious? | Caused by genetic, metabolic, nutritional, degenerative, toxic, environmental, immune, traumatic, or other non-transmissible processes. |
Contagious? | An infectious disease that can spread directly or indirectly between hosts. Not every infectious disease is contagious. |
Major Disease Categories
Infectious: Viruses, bacteria, fungi, parasites and prions. Examples: influenza, strep throat, tuberculosis, malaria.
Genetic / Congenital: Inherited mutations or developmental abnormalities. Examples: cystic fibrosis, sickle-cell disease, some congenital heart defects.
Metabolic / Endocrine: Disruption of metabolism or hormone regulation. Examples: diabetes mellitus, thyroid disorders.
Nutritional: Deficiency, excess, or imbalance of nutrients. Examples: iron-deficiency anemia, scurvy.
Degenerative: Progressive deterioration of cells, tissues, or organs. Examples: osteoarthritis and several neurodegenerative disorders.
Autoimmune / Immune-mediated: The immune system attacks or abnormally reacts to the body's own tissues or otherwise causes damaging inflammation.
Neoplastic: Abnormal uncontrolled cell growth; includes benign tumors and cancers.
Toxic / Environmental: Exposure to harmful chemicals, radiation, pollutants, heat, cold, or other environmental hazards.
Traumatic: Physical injury that damages tissues or organs.
Idiopathic: A disease whose cause remains unknown after appropriate evaluation.
Infectious Disease: The Chain of Infection
Agent → Reservoir → Portal of exit → Mode of transmission → Portal of entry → Susceptible host
Breaking any link can reduce transmission. Common interventions include hand hygiene, sanitation, ventilation, vaccination when available, safe food and water, vector control, isolation when appropriate, and personal protective equipment.
Common Modes of Transmission
- Direct contact — person-to-person physical contact.
- Indirect contact — contaminated objects or surfaces.
- Droplet — relatively large respiratory particles at close range.
- Airborne — smaller infectious particles that can remain suspended in air.
- Food or water — ingestion of contaminated material.
- Vector-borne — transmission by organisms such as mosquitoes or ticks.
- Blood/body fluids — exposure through blood or certain other body fluids.
- Vertical — parent-to-child transmission during pregnancy, birth, or breastfeeding.
Signs, Symptoms & Diagnosis
Symptom: a problem experienced and reported by the patient, such as pain, nausea, or fatigue. Sign: an observable or measurable finding, such as fever, rash, abnormal blood pressure, or a laboratory abnormality.
Diagnosis usually combines history, physical examination, epidemiologic context, and—when needed—laboratory tests, imaging, pathology, or other specialized studies.
Treatment vs. Control
Cure: eliminates the disease or its cause when that is possible.
Disease-directed treatment: targets the underlying mechanism or causative agent.
Supportive treatment: maintains hydration, oxygenation, nutrition, comfort, or other body functions while recovery occurs.
Symptom control: reduces symptoms without necessarily removing the underlying cause.
Prevention: reduces the probability that disease develops or spreads.
Antimicrobial Basics
Antibiotics treat susceptible bacterial infections; they do not treat viral infections. Antivirals, antifungals, and antiparasitic drugs target particular organisms and must be selected for the specific infection. Unnecessary or incorrect antimicrobial use promotes resistance and can cause adverse effects.
Prevention — Three Levels
Primary prevention: prevents disease before it occurs: vaccination, sanitation, exercise, nutrition, smoking avoidance, and exposure reduction.
Secondary prevention: detects disease early: screening, case finding, and early diagnostic testing.
Tertiary prevention: reduces complications or disability after disease is established: treatment, rehabilitation, and long-term disease management.
Useful Epidemiology Terms
Term | Meaning |
|---|---|
Incidence | New cases occurring in a population during a specified period. |
Prevalence | Existing cases in a population at a particular time or during a period. |
Morbidity | Illness, complications, or impaired health associated with disease. |
Mortality | Deaths in a population, often expressed as a rate. |
Endemic | Disease consistently present at an expected level in a population or area. |
Epidemic | Cases occurring above the expected level in a population or area. |
Pandemic | An epidemic that spreads across countries or continents and affects many people. |
Zoonosis | An infection naturally transmitted between vertebrate animals and humans. |
Red Flags
Seek urgent medical evaluation for severe difficulty breathing, chest pain or pressure, new confusion, fainting, signs of stroke, uncontrolled bleeding, severe dehydration, rapidly worsening illness, or other symptoms that appear life-threatening. The appropriate response depends on the individual and the disease.
Bottom Line
Disease is not one thing. It is a broad family of abnormal biological processes. The useful first questions are: What system is affected? Is the process infectious or noninfectious? What is the likely cause or mechanism? How is the diagnosis established? Can it be prevented, cured, controlled, or its complications reduced?