Principles of Disease and Epidemiology: Study Notes

Pathology, Infection, and Disease

Key Concepts in Pathology

• Pathology is the study of disease, including its causes, development, and effects on the body.

• Etiology refers to the cause of a disease, such as a specific microorganism or environmental factor.

• Pathogenesis describes how a disease develops and progresses within the host.

• Infection is the invasion or colonization of the body by pathogenic microorganisms.

• An infectious disease occurs when an infection causes a change from a healthy state to a diseased state.

Human Microbiome

Development and Role of the Microbiome

• The human microbiome begins developing before birth and continues to change throughout life.

• Normal microbiota are microorganisms that permanently colonize the host without causing disease under normal conditions.

• Transient microbiota are present for days, weeks, or months and then disappear.

• Normal microbiota contribute to both health (e.g., by preventing pathogen colonization) and disease (if balance is disrupted).

• Factors affecting microbiota include diet, age, hygiene, and environment.

Normal Microbiota and the Host

Interactions and Effects

• Microbial antagonism (competitive exclusion) is when normal microbiota compete with pathogens for nutrients and space.

• Normal microbiota may produce substances (e.g., bacteriocins) harmful to invading microbes.

• Clostridium difficile infections often occur after antibiotic treatment disrupts normal microbiota.

• Fecal microbial transfer (FMT) may help restore normal microbiota in patients with recurrent infections.

Symbiotic Relationships

Types of Symbiosis

• Commensalism: One organism benefits, the other is unaffected.

• Mutualism: Both organisms benefit.

• Parasitism: One organism benefits at the expense of the other (the host).

Classifying Infectious Diseases

Definitions and Examples

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

• Signs: Objective changes that can be observed or measured (e.g., fever, rash).

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

• Communicable diseases: Spread from one host to another (e.g., influenza).

• Noncommunicable diseases: Not spread from host to host (e.g., tetanus).

Occurrence of Disease

Incidence, Prevalence, and Patterns

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

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

• Sporadic disease: Occurs only occasionally (e.g., typhoid fever in the U.S.).

• Endemic disease: Constantly present in a population (e.g., common cold).

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

Severity and Duration of Disease

Types of Disease Progression

• Acute disease: Develops rapidly, lasts a short time (e.g., influenza).

• Chronic disease: Develops slowly, may last for a long period (e.g., tuberculosis).

• Latent disease: Pathogen remains inactive for a time, then becomes active (e.g., herpes simplex virus).

• Herd immunity: When most of a population is immune, protecting those who are not immune.

Extent of Host Involvement

Types of Infections

• Local infection: Limited to a small area of the body (e.g., boil).

• Systemic infection: Spread throughout the body via blood or lymph (e.g., measles).

• Focal infection: Begins as a local infection, spreads to other areas (e.g., from teeth to heart).

• Bacteremia: Presence of bacteria in the blood.

• Septicemia: Growth of bacteria in the blood (blood poisoning).

• Viremia: Presence of viruses in the blood.

Primary and Secondary Infections

Infection Sequence

• Primary infection: The initial infection in a healthy individual.

• Secondary infection: Follows a primary infection, often by an opportunistic pathogen.

• Subclinical infection: No noticeable signs or symptoms (asymptomatic).

Predisposing Factors

Factors Increasing Disease Susceptibility

• Predisposing factors increase susceptibility to disease.

• Examples: Age, nutrition, genetic background, vaccination status, climate, fatigue, lifestyle, and pre-existing illnesses.

Development of Disease

Stages of Disease Progression

• Incubation period: Interval between infection and appearance of first symptoms.

• Prodromal period: Short period with mild, nonspecific symptoms.

• Period of illness: Disease is most severe; signs and symptoms are most evident.

• Convalescence: Body returns to its pre-diseased state; recovery occurs.

Reservoirs of Infection

Sources of Pathogens

• Reservoirs: Continual sources of infection (human, animal, or nonliving).

• Asymptomatic carriers: Individuals who harbor pathogens but show no signs of disease.

• Zoonoses: Diseases transmitted from animals to humans (e.g., rabies, Lyme disease).

• Soil and water can act as nonliving reservoirs (e.g., Clostridium botulinum in soil).

Transmission of Disease

Modes of Transmission

• Direct contact transmission: Requires close physical contact between hosts (e.g., touching, kissing).

• Indirect contact transmission: Involves a nonliving object (fomite) such as doorknobs, utensils.

• Droplet transmission: Spread via airborne droplets less than 1 meter in distance (e.g., coughing, sneezing).

• Vehicle transmission: Transmission by a medium such as air, food, or water.

• Cross-contamination: Transfer of pathogens from one food or object to another.

Vectors

Role in Disease Transmission

• Vectors: Animals (often arthropods like mosquitoes and ticks) that transmit pathogens between hosts.

• Mechanical transmission: Pathogens are carried on the vector's body surface.

• Biological transmission: Pathogens reproduce within the vector and are transmitted via bites or feces.

Healthcare-Associated Infections (HAIs)

Nosocomial Infections

• Healthcare-associated infections (HAIs) are also called nosocomial infections.

• HAIs are acquired while receiving treatment in a healthcare facility (e.g., hospital, clinic).

• Clostridioides difficile is a leading cause of HAIs, especially after antibiotic use.

• Compromised hosts have impaired resistance to infection due to disease, therapy, or burns.

Table: Types of Disease Transmission

Transmission Mode	Description	Example
Direct Contact	Physical contact between hosts	Touching, kissing
Indirect Contact	Via nonliving object (fomite)	Doorknobs, utensils
Droplet	Airborne droplets <1 meter	Coughing, sneezing
Vehicle	Transmission by air, food, water	Contaminated water
Vector (Mechanical)	Pathogen carried on body surface	Housefly landing on food
Vector (Biological)	Pathogen reproduces in vector	Malaria via mosquito bite

Table: Types of Infections

Type	Description	Example
Local	Confined to small area	Boil
Systemic	Spread throughout body	Measles
Focal	Starts local, spreads	Dental infection spreading to heart
Bacteremia	Bacteria in blood	Transient bacteremia after dental work
Septicemia	Bacterial growth in blood	Sepsis
Viremia	Viruses in blood	HIV viremia