Principles of Disease and Epidemiology: Study Guide Notes

Principles of Disease

Types of Symbiosis

Symbiosis refers to the close association between two different species. In microbiology, understanding symbiotic relationships helps explain how microbes interact with hosts.

• Mutualism: Both organisms benefit (e.g., Escherichia coli in the human gut synthesizes vitamins).

• Commensalism: One organism benefits, the other is unaffected (e.g., skin microbiota).

• Parasitism: One organism benefits at the expense of the other (e.g., pathogenic bacteria causing disease).

Normal Microbiota

The normal microbiota consists of microorganisms that reside on or within the human body without causing disease under normal conditions.

• Resident microbiota: Permanent members of the body’s microbial community.

• Transient microbiota: Temporary members that may be present for days, weeks, or months.

• Acquisition: Microbiota are acquired during birth, from the environment, and through contact with caregivers.

• Functions: Compete with pathogens, aid digestion, synthesize vitamins, and stimulate the immune system.

Opportunistic Infections

Normal microbiota can cause disease under certain conditions:

• Immune suppression (e.g., HIV/AIDS)

• Changes in normal microbiota (e.g., antibiotic use)

• Introduction into unusual body sites (e.g., Staphylococcus aureus entering wounds)

Reservoirs of Infection

Reservoirs are sources where pathogens persist and can be transmitted.

• Human reservoirs: Infected individuals (symptomatic or asymptomatic carriers)

• Animal reservoirs: Zoonoses (e.g., rabies from dogs)

• Nonliving reservoirs: Soil, water, and food (e.g., Clostridium tetani in soil)

Portals of Entry and Exit

Pathogens enter and exit the body through specific portals:

• Entry: Skin, mucous membranes, placenta, parenteral route (e.g., cuts)

• Exit: Respiratory tract, gastrointestinal tract, genitourinary tract, blood

Key Terms in Infectious Disease

• Infection: Invasion and multiplication of pathogens in the body.

• Disease: Any change from a state of health; results from infection.

• Morbidity: Incidence of disease in a population.

• Pathogenicity: Ability of a microbe to cause disease.

• Virulence: Degree of pathogenicity.

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

• Symptoms: Subjective changes felt by the patient (e.g., pain).

Biofilms and Infection

Biofilms are communities of microorganisms attached to surfaces, encased in a protective matrix.

• Facilitate contamination and infection by protecting microbes from immune responses and antibiotics.

• Common in medical devices (e.g., catheters).

Koch's Postulates

Koch’s postulates are criteria used to establish a causative relationship between a microbe and a disease.

• 1. The microorganism must be found in all cases of the disease.

• 2. It must be isolated and grown in pure culture.

• 3. The cultured microorganism must cause disease when introduced into a healthy host.

• 4. It must be re-isolated from the experimentally infected host.

• Limitations: Not all microbes can be cultured; some diseases are caused by multiple organisms.

• Etiology: The study of the cause of disease.

Virulence Factors

Virulence factors are traits that enable microbes to cause disease.

• Extracellular enzymes: Break down host tissues (e.g., collagenase).

• Toxins: Poisonous substances (e.g., exotoxins, endotoxins).

• Adhesion factors: Allow attachment to host cells (e.g., fimbriae).

• Antiphagocytic factors: Prevent phagocytosis (e.g., capsules).

Stages of Infectious Disease

Infectious diseases typically progress through five stages:

• 1. Incubation period

• 2. Prodromal period

• 3. Illness

• 4. Decline

• 5. Convalescence

Modes of Transmission

Pathogens are transmitted by various modes:

• Contact transmission: Direct, indirect, or droplet

• Vehicle transmission: Air, water, food

• Vector transmission: Mechanical (passive) or biological (active)

• Droplet vs Airborne: Droplet transmission involves large particles over short distances; airborne involves small particles over long distances.

• Mechanical vs Biological Vectors: Mechanical vectors carry pathogens on their bodies; biological vectors transmit pathogens through bites.

Classification of Diseases

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

• Subacute: Intermediate duration

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

• Latent: Pathogen remains inactive for a period (e.g., herpesvirus)

• Communicable: Can be spread from person to person

• Noncommunicable: Not spread between hosts

Epidemiology

Emerging and Re-emerging Infectious Diseases

Factors contributing to emerging diseases include microbial adaptation, human behavior, and environmental changes.

Definition and Principles

Epidemiology is the study of the distribution and determinants of health-related states in populations.

• Used to track disease outbreaks, identify risk factors, and develop control strategies.

• Methods include surveillance, data analysis, and field investigations.

Epidemiological Curves

Epidemiological curves (epi curves) graph the number of cases over time to identify patterns of disease spread.

• Point-source outbreak: Sharp rise and fall in cases.

• Continuous common source: Cases rise and remain elevated.

• Propagated outbreak: Gradual rise as disease spreads person-to-person.

Incidence vs Prevalence

• Incidence: Number of new cases in a given time period.

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

Example: If 10 new cases of measles occur in a month, incidence is 10; if 50 people currently have measles, prevalence is 50.

Pandemic, Epidemic, and Endemic

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

• Epidemic: Sudden increase in cases in a region

• Endemic: Constant presence of a disease in a population

Public Health

Public health focuses on protecting and improving the health of populations.

• Three core functions:

  1. Assessment

  2. Policy development

  3. Assurance

Nosocomial (Hospital-Acquired) Infections

Nosocomial infections are acquired in healthcare settings.

• Contributing factors: Immunocompromised patients, invasive procedures, antibiotic use, poor hygiene.

• Prevention: Hand hygiene, sterilization, isolation protocols, surveillance.

Summary Table: Types of Disease Transmission

Transmission Type	Description	Example
Contact (Direct)	Physical contact between hosts	Touching, sexual contact
Contact (Indirect)	Via fomites (objects)	Doorknobs, towels
Droplet	Large respiratory droplets	Coughing, sneezing
Airborne	Small particles over distance	Tuberculosis
Vehicle	Through air, water, food	Cholera (water)
Vector (Mechanical)	Passive transport by vector	Flies carrying pathogens
Vector (Biological)	Active transmission via bite	Malaria (mosquito)

Key Equations

• Incidence Rate:

$\text{Incidence Rate} = \frac{\text{Number of new cases}}{\text{Population at risk} \times \text{Time period}}$

• Prevalence Rate:

$\text{Prevalence Rate} = \frac{\text{Total number of cases}}{\text{Total population}}$

Additional info: Academic context and examples have been added to expand upon the brief learning objectives and provide a self-contained study guide.