Principles of Disease and Epidemiology

Principles of Disease and Epidemiology, Part I

Pathology, Infection, and Disease

  • Pathology: The study of disease.
  • Etiology: The study of the cause of a disease.
  • Disease: An abnormal state in which the body is not functioning normally (e.g., diabetes).
  • Infection: Colonization of the body by pathogens.
  • Pathogenesis: The development of disease.
  • Infectious Disease: A disease that can be transmitted between hosts.

Normal Microbiota and the Host

  • Transient microbiota: Present for days, weeks, or months.
  • Normal microbiota: Permanently colonize the host.
  • Symbiosis: The relationship between normal microbiota and the host, which can have:
    • Commensalism: One organism benefits, the other is unaffected.
    • Mutualism: Both organisms benefit.
    • Parasitism: One organism benefits at the expense of the other.
  • Opportunistic pathogens: Normal microbiota that can become pathogenic under certain conditions.

Normal Microbiota on the Human Body

  • Common sites:
    • Nose and throat (upper respiratory system)
    • Eyes (conjunctiva)
    • Mouth
    • Skin
    • Large intestine
    • Urinary and reproductive systems
  • Microbial antagonism: Competition between microbes for resources.
  • Probiotics: Live microbes that exert a beneficial effect when applied to or ingested by the body.

Koch's Postulates

  • Purpose: Proves that a certain microorganism causes a specific disease.
  • Criteria:
    1. Agent X must be present in diseased hosts and absent in healthy ones.
    2. The agent must be isolated and grown in pure culture.
    3. Introduction of the agent into a healthy host must induce the disease.
    4. The agent should be re-isolated from the new diseased host, confirming it's the same pathogen.

Complications of Koch's Postulates

  • Some pathogens cause multiple diseases.
  • Some affect only humans, so animal models are inappropriate.
  • Not all individuals may exhibit symptoms despite being carriers (e.g., MRSA).

Classifying Infectious Diseases

  • Symptom: A change felt by a patient (e.g., pain).
  • Sign: An observable change (e.g., fever).
  • Syndrome: A group of symptoms/signs that occur together (e.g., chronic fatigue syndrome).
  • Disease categories:
    • Communicable disease: Spreadable from host to host.
    • Contagious disease: Easily spreadable.
    • Noncommunicable disease: Not transmitted between hosts.

Severity and Duration of Disease

  • Acute disease: Rapid onset, resolves quickly (e.g., Norovirus).
  • Chronic disease: Slow onset, long-lasting (e.g., brucellosis).
  • Latent disease: Inactive phase before symptoms appear (e.g., herpes).

Extent of Host Involvement

  • Local infection: Confined to a specific area (e.g., UTI).
  • Systemic infection: Spread throughout the body.
  • Sepsis: Toxic inflammatory illness from spread of microbes.
  • Primary infection: Initial acute infection.
  • Secondary infection: Follows a primary infection.

Predisposing Factors

  • Factors that make the body more susceptible to disease include:
    • Inherited traits
    • Climate and weather
    • Lifestyle factors (e.g., hygiene access)
    • Age
    • Chemotherapy treatments affecting the immune system.

Emerging Infectious Diseases

  • Characteristics:
    • New, increasing in incidence, or potential for increased occurrence.
    • Contributing factors include species jumping (e.g., HIV from primates) and genetic recombination (e.g., E. coli).
  • Recent examples include SARS-CoV-2 and Hantavirus.

Reservoirs of Infection

  • Human: carriers of diseases (e.g., herpes).
  • Animal: zoonotic diseases (e.g., rabies).
  • Nonliving: Tetanus from contaminated soil.

Transmission of Disease

  • Contact:
    • Direct contact: Close interaction between hosts.
    • Indirect contact: Through contaminated objects (fomites).
    • Droplet transmission: Airborne particles.
  • Vectors: Arthropods transmitting pathogens through mechanical or biological means.

Epidemiology Overview

  • Descriptive epidemiology: Collection & analysis of data (John Snow, cholera).
  • Analytical epidemiology: Comparing diseased to healthy groups (Florence Nightingale).
  • Experimental epidemiology: Control experiments to test hypotheses.

Occurrence of Disease**

  • Incidence: New cases within a specific time frame.
  • Prevalence: Total cases at a specific time.
  • Sporadic disease: Occasional outbreaks.
  • Endemic disease: Constant presence in a population.
  • Epidemic disease: Significant outbreaks contributing to widespread infection.
  • Pandemic: Global epidemic such as COVID-19.

Reporting and Tracking Diseases

  • Case reporting is essential for tracking spread and incidence.
  • Morbidity: Incidence of a specific disease.
  • Mortality: Death rate from a disease.

Nosocomial Infections

  • Infections acquired in healthcare settings.
  • Common types:
    • Urinary tract infections
    • Surgical site infections
    • Lower respiratory infections
    • Bloodstream infections
  • Prevention strategies include cleanliness and surveillance.