Principles of Disease & Epidemiology

Overview

This chapter introduces the foundational concepts of pathology, infectious disease cycles, human microbiota, and epidemiological principles. It covers the mechanisms by which diseases develop, how they are classified, and the factors influencing their spread and severity.

Pathology, Infection, & Disease

Key Definitions

• Pathology: The scientific study of disease, focusing on its causes, development, and effects on the body.

• Etiology: The cause of a disease.

• Pathogenesis: The manner in which a disease develops.

• Bodily Changes: Structural and functional changes resulting from disease.

• Infection: The invasion and multiplication of microorganisms in the body.

• Disease: A condition in which normal body functions are impaired due to infection or other causes.

• Virulence: The degree of pathogenicity of a microorganism.

• Pathogenicity: The ability of a microorganism to cause disease.

Infectious Disease Cycle

Stages of Disease Transmission

• Reservoir: The source of the microbial pathogen (e.g., humans, animals, environment).

• Transmission: The movement of pathogens from the reservoir to a susceptible host.

• Susceptible Host: An individual who can be infected.

• Microbial Pathogenesis: The process by which pathogens enter the host, overcome barriers, invade tissues, and cause damage.

• Virulence Factors: Molecules produced by pathogens that enhance their ability to cause disease.

Human Microbial Flora

Normal Microbiota

• The human body is colonized by millions of bacteria, collectively termed the normal microbiota.

• Microbes colonize non-sterile areas exposed to the environment (skin, oral/nasal cavities, lungs).

• Transient microbiota: Microbial species present for short periods.

• Presence of microbes in sterile areas (e.g., blood, cerebrospinal fluid) signals infection.

• Factors affecting colonization include both physical and chemical conditions.

Host: Normal Microbiota Relationship

Microbial Antagonism & Symbiosis

• Normal microbiota exhibit microbial antagonism (competitive exclusion), preventing colonization by pathogens.

• They compete for nutrients and space, produce antimicrobial agents, and modify their environment.

• Disruption of this balance can result in disease.

• Types of symbiotic relationships:

  • Commensalism (+/0): One organism benefits, the other is unaffected. Example: Bacteria on skin.

  • Mutualism (+/+): Both organisms benefit. Example: Escherichia coli in the gut synthesizing vitamins.

  • Parasitism (+/-): One organism benefits at the expense of the other. Example: Pathogenic bacteria causing disease.

• Opportunistic pathogens can cause disease when host defenses are compromised.

• Asymptomatic carriers: Individuals who harbor pathogens without showing symptoms.

Etiology of Infectious Diseases: Koch’s Postulates

Steps to Identify Disease Causation

1. The same pathogen must be present in every case of the disease.

2. The pathogen must be isolated from the diseased host and grown in pure culture.

3. The cultured pathogen must cause the disease when inoculated into a healthy, susceptible host.

4. The pathogen must be re-isolated from the newly diseased host and shown to be the same as the original.

Exceptions to Koch’s Postulates

• Some healthy individuals are carriers (e.g., Treponema pallidum for syphilis).

• Some microbes cannot be cultured in artificial media (e.g., Legionella pneumophila).

• Similar symptoms can be caused by different pathogens (e.g., pneumonia, meningitis).

• The same pathogen can cause different diseases (e.g., Streptococcus pyogenes: strep throat, scarlet fever).

• Viral agents may lack suitable animal models or have latent forms.

Classifying Infectious Diseases

Symptoms, Signs, and Syndromes

• Symptoms: Subjective effects felt by the patient (e.g., malaise, headache).

• Signs: Objective, measurable changes (e.g., fever, rash).

• Syndrome: A group of signs and symptoms that characterize a disease.

Types of Diseases

• Communicable disease: Can be spread from host to host (e.g., measles, flu).

• Contagious disease: Easily spread from person to person (e.g., cold, pink eye).

• Non-communicable disease: Not spread from host to host (e.g., tetanus).

Occurrence of Disease

Incidence and Prevalence

• Incidence: Number of new cases in a population during a specific time period. Formula:

• Prevalence: Total number of cases (new and existing) at a specified time. Formula:

• Both metrics help estimate disease spread and impact.

Frequency & Severity of Disease

Frequency of Occurrence

• Sporadic: Disease occurs infrequently.

• Endemic: Disease is always present in a population (e.g., common cold).

• Epidemic: Rapid increase in cases in a specific area.

• Pandemic: Worldwide epidemic (e.g., COVID-19).

Severity or Duration

• Acute disease: Rapid onset, short duration (e.g., influenza).

• Chronic disease: Slow development, long duration (e.g., tuberculosis).

• Subacute disease: Intermediate between acute and chronic.

• Latent disease: Periods of inactivity (e.g., shingles).

Herd Immunity

Concept and Application

• Immune individuals act as barriers to disease spread, protecting non-immune individuals.

• Achieved through vaccination or previous infection.

• Does not apply to non-communicable diseases.

• R0 value: Average number of people infected by one person with the disease.

Classification by Extent of Infection

• Local infection: Confined to a specific area.

• Systemic infection: Spreads throughout the body via circulatory/lymphatic systems.

• Focal infection: Starts local, spreads to specific body parts (e.g., teeth, tonsils).

• Sepsis: Inflammatory response due to toxins/bacteria in blood; septicemia is a severe form.

State of Host Resistance

• Primary infection: Initial, acute illness.

• Secondary infection: Caused by opportunistic pathogens after primary infection.

• Subclinical (inapparent) infection: No noticeable symptoms.

Predisposing Factors & Disease Development

Factors Increasing Susceptibility

• Sex (female vs. male)

• Inherited traits (e.g., sickle cell gene)

• Climate and weather

• Fatigue

• Age

• Lifestyle

• Chemotherapy

Stages in Disease Development

1. Incubation period: Time between exposure and appearance of symptoms.

2. Prodromal period: Early, mild symptoms.

3. Period of illness: Most severe signs and symptoms.

4. Period of decline: Signs and symptoms subside.

5. Period of convalescence: Recovery and return to normal function.

Summary Table: Key Concepts

Additional info: R0 values and herd immunity thresholds are subject to change based on new epidemiological data.