Principles of Disease and Epidemiology: Study Notes

Pathology, Infection, and Disease

Key Concepts in Pathology

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

• Etiology refers to the specific cause or origin of a disease (e.g., a particular bacterium or virus).

• Pathogenesis describes the mechanisms by which a disease develops and progresses in the host.

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

• An infectious disease results when an infection leads to a change from a healthy state.

Human Microbiome

Development and Role of Microbiota

• The human microbiome begins to develop at birth and consists of all the microorganisms that inhabit the human body.

• Normal microbiota are permanent residents of the body and usually do not cause disease under normal conditions.

• Transient microbiota are present temporarily (days to months) and may be removed by the host's defenses or competition.

• Normal microbiota can contribute to both health (e.g., by preventing pathogen colonization) and disease (e.g., if displaced or if the host is compromised).

• Factors influencing microbiota include diet, age, hygiene, and geographic location.

Normal Microbiota and the Host

Microbial Interactions

• Microbial antagonism (competitive exclusion) is the process by which normal microbiota inhibit the growth of pathogens by competing for nutrients and producing harmful substances.

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

• Fecal microbial transfer (FMT) can help restore normal microbiota in patients with recurrent C. difficile infection.

Symbiotic Relationships

Types of Symbiosis

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

• Mutualism: Both organisms benefit (e.g., Escherichia coli in the gut synthesizes vitamins for the host and receives nutrients).

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

Classifying Infectious Diseases

Definitions and Examples

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

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

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

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

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

Occurrence of Disease

Measuring and Describing Disease

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

• Prevalence: Total number of cases (new and existing) in a population 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: A 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 years (e.g., tuberculosis).

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

• Herd immunity: Occurs when a large portion of a population is immune, reducing disease spread.

Extent of Host Involvement

Types of Infections

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

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

• Focal infection: Begins locally, spreads to other parts (e.g., dental infection leading to endocarditis).

• Bacteremia: Presence of bacteria in the blood.

• Septicemia: Active bacterial growth 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: Occurs after the primary infection, often due to weakened defenses (e.g., pneumonia after influenza).

• Subclinical infection: No noticeable signs or symptoms (e.g., asymptomatic COVID-19).

Predisposing Factors

Factors Increasing Disease Susceptibility

• Include age, nutritional status, vaccination status, stress, genetic background, and pre-existing illnesses.

Development of Disease

Stages of Disease Progression

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

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

• Period of illness: Disease is most severe; characteristic signs and symptoms appear.

• Convalescence: Recovery period; body returns to normal state.

Reservoirs of Infection

Sources of Pathogens

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

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

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

• Nonliving reservoirs: Soil and water can harbor pathogens (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 or utensils.

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

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

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

Vectors

Role of Arthropods in Disease Transmission

• Vectors: Usually arthropods (e.g., mosquitoes, ticks) that transmit pathogens.

• Mechanical transmission: Pathogen is carried on the vector's body surface (e.g., housefly transferring bacteria from feces to food).

• Biological transmission: Pathogen reproduces within the vector and is transmitted via bite or feces (e.g., malaria via Anopheles mosquitoes).

Healthcare-Associated Infections (HAIs)

Nosocomial Infections

• Healthcare-associated infections (HAIs) are acquired while receiving treatment in a healthcare facility.

• Also called nosocomial infections.

• Clostridioides difficile is a leading cause of antibiotic-associated diarrhea in hospitals.

• Compromised hosts have impaired resistance due to disease, therapy, or age, increasing susceptibility to HAIs.

Summary Table: Types of Disease Transmission

Transmission Mode	Description	Example
Direct Contact	Physical contact between hosts	Touching, kissing
Indirect Contact	Via fomites (nonliving objects)	Contaminated doorknobs
Droplet	Short-range airborne droplets (<1 m)	Coughing, sneezing
Vehicle	Transmission by air, food, or water	Contaminated water
Vector (Mechanical)	Pathogen carried on vector's body	Housefly on food
Vector (Biological)	Pathogen reproduces in vector	Malaria via mosquito

Key Equations

• Incidence Rate: $\text{Incidence} = \frac{\text{Number of new cases during a time period}}{\text{Total population at risk}}$

• Prevalence Rate: $\text{Prevalence} = \frac{\text{Number of total cases (old and new)}}{\text{Total population}}$

Example: If a city of 100,000 people has 500 new cases of influenza in January, the incidence is $\frac{500}{100,000} = 0.005$ or 0.5% for that month.

Additional info: The above notes expand on the original outline by providing definitions, examples, and context for each concept, as well as a summary table and relevant epidemiological equations.