Principles of Disease and Epidemiology

Symbiosis

• Definition: Life together; relationship between normal flora and host.

Types of Symbiosis

• Commensalism: One organism benefits, the other is unaffected.
  • Examples: Microorganisms on eyes and external genitalia.
• Mutualism: Both organisms benefit.
  • Example: E. coli in large intestine synthesizes vitamins K and B.
• Parasitism: One organism benefits at the expense of the other.
  • Example: Pathogens causing disease.

Normal Flora

• Definition: Microorganisms colonizing the host without causing disease under normal conditions.
• Role: Provide protection by microbial antagonism.

Definitions in Disease

Pathology

• Definition: Scientific study of disease.

Etiology

• Definition: Cause of disease.

Pathogenesis

• Definition: Origin and development of disease.

Virulence

• Definition: Degree of pathogenicity.
• Factors: Molecules produced by pathogens, e.g., capsule formation, toxin production.

Koch's Postulates

• Used to determine the etiology of infection.

Disease Principles

Colonization vs. Infection

• Colonization: Presence and growth without clinical signs.
• Infection: Invasion with clinical signs like inflammation and fever.

Disease Signs and Symptoms

• Signs: Discovered by physician.
• Symptoms: Reported by patient.
• Syndrome: Association of signs and symptoms.

Primary vs. Secondary Infection

• Primary: Acute initial illness.
• Secondary: Occurs during/after treatment of another infection.

Pathogens

• Primary Pathogen: Causes disease in a healthy host.
• Opportunistic Pathogen: Causes disease in immunocompromised hosts.

Occurrence of Disease

Incidence vs. Prevalence

• Incidence: New cases in a population during a specific time.
• Prevalence: Total cases in a population at a specific time.

Endemic, Epidemic, Pandemic

• Endemic: Constantly present, e.g., common cold.
• Epidemic: Many acquire in short time, e.g., influenza.
• Pandemic: Worldwide epidemic, e.g., COVID-19.

Disease Duration

• Acute: Rapid development, short duration.
• Chronic: Slow development, long duration.
• Latent: Inactive period, then reactivation (e.g., shingles).

Infections

Local vs. Systemic

• Local: Limited to a small area.
• Systemic: Spread throughout the body.

Blood Infections

• Bacteremia: Bacteria in blood.
• Viremia: Virus in blood.
• Toxemia: Toxins in blood.
• Septicemia: Blood poisoning.

Disease Progression Stages

1. Incubation: Initial exposure.
2. Prodromal: Mild symptoms.
3. Illness: Severe symptoms.
4. Convalescence: Recovery.

Disease Reservoirs

• Human Reservoirs: Symptomatic or asymptomatic.
• Animal Reservoirs: Zoonotic infections.
• Environmental Reservoirs: Soil and water.

Transmission of Infectious Disease

Communicable vs. Non-Communicable

• Communicable: Spread from host to host (e.g., influenza).
• Contagious: Easily transmissible (e.g., chickenpox).
• Non-Communicable: Not spread host to host (e.g., cancer).

Contact Transmission

• Direct Contact: Physical contact.
• Indirect Contact: Via fomites.
• Droplet Transmission: Respiratory droplets.

Vehicle Transmission

• Water, Food, Airborne: Spread through these mediums.

Vectors

• Mechanical Vectors: Carry pathogens on body.
• Biological Vectors: Pathogens reproduce within vector.

Nosocomial Infections

• Definition: Hospital-acquired infections.
• Prevention: Handwashing.
• Sources: Patient flora, hospital environment.

Emerging Infectious Diseases

• Factors: Antibiotic resistance, environmental changes, population behavior.
• Examples: SARS, avian influenza.

Epidemiology

Key Figures

• John Snow: Father of Epidemiology.
• Ignaz Semmelweis: Advocated hand-washing.
• Florence Nightingale: Improved sanitary conditions.

Types of Epidemiology Studies

• Descriptive: Occurrence of disease.
• Analytical: Analyzes trends.
• Experimental: Tests hypotheses.
• Case Reporting: Reporting specific cases.

Morbidity vs. Mortality

• Morbidity: Incidence of disease.
• Mortality: Deaths from disease.

Conclusion

• For further information on infectious disease surveillance, resources are available at the county, state, national, and world levels.