Principles of Disease and Epidemiology

Chapter 14: Principles of Disease and Epidemiology

Pathology, Infection, and Disease

  • Pathology

    • Defined as the study of disease caused by pathogens.

    • Focuses on the relationship between the pathogen and host.

  • Etiology

    • Definition: The study of the cause (origin) of a disease.

    • A disease's cause is often referred to as its "causative agent."

  • Pathogenesis

    • Definition: The development of disease, involving observable structural and functional changes in the host.

  • Infection

    • Defined as the invasion or colonization of the body by pathogens.

    • Infection can lead to disease, though it does not always result in disease.

  • Disease

    • Definition: An abnormal state in which the body is not performing normal functions.

    • Types of diseases:

    1. Infectious Disease

      • Involves pathogens.

    2. Non-infectious Disease

      • Examples include diabetes, cancer, hypertension (HTN).

Normal Microbiota

  • Normal Microbiota

    • Generally harmless to the host if they remain in a specific body area.

  • Resident Microbiota

    • Always present in and on the human host.

  • Transient Microbiota

    • Temporary residents in and on the human host; remain for only hours to months.

    • Can be eliminated through washing hands or bathing; typically harmless.

    • Often found on the skin.

Microbe and Host Relationships

  • Symbiosis

    • Defined as the relationship between microbe and host; identifying who benefits and who does not.

Types of Symbiosis

  1. Commensalism

    • One organism benefits while the other is unaffected.

    • Example: Bacteria on human skin.

  2. Mutualism

    • Both organisms benefit from the relationship.

    • Example: E. coli in the colon produces Vitamin K.

  3. Parasitism

    • One organism benefits at the expense of the other.

    • Example: Influenza viruses infect host cells, acting as an "obligate intracellular parasite."

    • Parasite is synonymous with pathogen; always causes harm to the host.

Koch's Postulates

  • Koch's Postulates

    • Framework for connecting a specific pathogen to a specific disease, thus affirming the Germ Theory of disease.

Four Criteria to Establish a Causative Relationship

  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 isolated pathogen from pure culture must cause disease when inoculated into a healthy host (laboratory animal).

  4. Must re-isolate the pathogen from the inoculated animal, confirming it to be the original organism.

  • Application of Koch's Postulates

    • Used to identify the etiology of diseases; serves as the first step in treatment and prevention.

    • Example Steps:

    1. Isolate microorganisms from a diseased or dead animal.

      • (e.g., Rat #1)

    2. Grow microorganisms in a pure culture to identify them.

    3. Inoculate another healthy lab animal (e.g., Rat #2) with the cultured microorganisms.

    4. Confirm reproduction of the disease in the lab animal.

    5. Isolate and grow the pathogen again in pure culture, leading to identification and comparison with the original pathogen.

Exceptions to Koch's Postulates

  1. Some pathogens can cause multiple disease conditions (not a 1:1 relationship).

    • Example: Streptococcus pyogenes causes strep throat and scarlet fever.

  2. Some pathogens only cause disease in humans and cannot be reproduced in lab animals.

  3. Some microbes cannot grow on artificial media and may require an animal host; these situations can yield false results if tested on lab media.

Classifying Infectious Diseases

  • Symptoms

    • Changes in body function felt by the patient.

    • Subjective experiences and cannot be measured (e.g., pain, fatigue).

  • Signs

    • Observable changes in the body, measurable aspects (e.g., blood pressure, temperature, weight).

  • Syndrome

    • A specific group of signs and symptoms that accompany a disease.

    • Examples include AIDS and Down syndrome.

  • Communicable Disease

    • Can be spread from one host to another.

    • Contagious Disease

    • Specifically describes diseases that are easily and rapidly spread (e.g., chicken pox).

  • Noncommunicable Disease

    • Not spread between hosts (e.g., tetanus, cancer, diabetes, autoimmune diseases).

Occurrence of a Disease (Frequency)

  • Incidence

    • Number of new cases developing during a specific time period (e.g., 1 year out of 10 years, or 1 month of 1 year).

  • Prevalence

    • Total number of cases at a given time, accounting for both new and old cases, indicating how widespread the disease is.

  • Sporadic Disease

    • Occurs occasionally with no predictable pattern (e.g., mad cow disease).

  • Endemic Disease

    • Disease constantly present in a localized population (e.g., malaria).

  • Epidemic Disease

    • Disease rapidly acquired by many people in a given area within a short time (e.g., influenza).

  • Pandemic Disease

    • Global epidemic affecting a larger geographical area (e.g., HIV/AIDS, COVID-19).

Severity or Duration of a Disease

  • Acute Disease

    • Symptoms develop rapidly but the disease lasts a short time (e.g., influenza).

  • Chronic/Persistent Disease

    • Symptoms develop slowly and the disease lasts a long time (e.g., tuberculosis).

  • Subacute Disease

    • Symptoms are intermediate between acute and chronic.

  • Latent Disease

    • The causative agent remains inactive for a period, activating later to produce symptoms; pathogens remain hidden/dormant (e.g., shingles, cold sores, genital herpes).

Herd Immunity

  • Defined as the immunity of a population that protects individuals who are not immune.

  • Achieved through vaccinations, where a majority of the population is immunized, consequently protecting the entire population, including non-immunized individuals.

  • Examples include chicken pox and measles.

Extent of Host Involvement

  • Local Infection

    • Pathogens are confined to a localized area and are not spread (e.g., localized skin infections).

  • Systemic (Generalized) Infection

    • Infection spreads throughout the body, often via the bloodstream.

Types of Systemic Infections

  • Sepsis/Septicemia

    • Life-threatening condition with uncontrollable spread of microbes in the blood; also referred to as blood infection.

    • Types include:

    • Bacteremia: Presence of bacteria in blood.

    • Toxemia: Presence of toxins in blood.

    • Viremia: Presence of virus in blood.

Extent of Host Involvement – Infections

  • Primary Infection

    • The initial infection that leads to illness; often leaves the host’s immune system weakened, predisposing the individual to secondary infections (e.g., HIV).

  • Secondary Infection

    • Infections caused by opportunistic pathogens that take advantage of a weakened immune system post-primary infection (e.g., opportunistic infections related to AIDS).

Predisposing Factors

  • Factors that increase susceptibility to disease include:

    • Gender: Females may be more susceptible to urinary tract infections (UTIs).

    • Age: Elderly and infants have weaker immune systems.

    • Lifestyle: IV drug use can increase vulnerability to infections.

    • Inherited Traits: E.g., an HIV-positive mother passing the disease to her baby.

    • Climate and Weather: Certain environments, such as tropics, promote the spread of diseases like malaria.

    • Poor Sanitary Conditions: Can lead to diseases like parasitic worm infections.

Development/Stages of Disease

  1. Incubation Period

    • No signs/symptoms present; interval between initial infection and the appearance of signs/symptoms. The pathogen is actively growing.

  2. Prodromal Period

    • Early, mild symptoms marking a brief period following incubation.

  3. Period of Illness

    • The time when observable signs and symptoms of disease are most severe; the body starts producing antibodies.

  4. Peak of Illness

    • The stage where the number of pathogens and antibodies is at its highest, leading to:
      i. Period of decline.
      ii. Death (if the immune system is overwhelmed).

  5. Period of Decline

    • Symptoms gradually decline, and the number of pathogens decreases.

  6. Period of Convalescence/Recovery

    • The body returns to its pre-diseased state; the patient regains strength. Pathogens are leaving the body; the patient may remain asymptomatic but still infectious, exhibiting chronic carrier status.

Reservoirs of Infection

  • A Reservoir is the continual source of infection, essentially the location where a pathogen can normally be found.

  • Three Categories of Reservoirs:

    1. Human Reservoirs

    • Pathogens typically found in humans, including both sick individuals and asymptomatic carriers who may not be aware they harbor the pathogen.

    1. Animal Reservoirs

    • Pathogens primarily found in animals.

    • Example: Zoonoses—ability to transmit pathogens from animals to humans.

    1. Non-living Reservoirs

    • Pathogens found in contaminated inanimate objects (e.g., soil, air, water, food).