Principles of Disease and Epidemiology: Study Notes

Chapter 14: Principles of Disease and Epidemiology

Pathology, Infection, and Disease

This section introduces the foundational concepts in the study of disease, focusing on definitions and distinctions between infection and disease.

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

• Etiology: The cause or origin of a disease.

• Pathogenesis: The process by which a disease develops and progresses.

• Infection: The colonization of the body by pathogenic microorganisms.

• Infectious Disease: A condition in which infection leads to abnormal body function.

• An infection may exist without causing detectable disease.

• Disease can result if a microorganism is present in an unusual location in the body.

Human Microbiome

The human microbiome consists of all the microorganisms that reside on and within the human body, playing a crucial role in health and disease.

• Microbiome establishment begins in utero (during pregnancy).

• The placental microbiome includes members of Enterobacteriaceae and Propionibacterium.

• Lactobacilli colonize the neonate's intestine during vaginal birth.

• Microorganisms are acquired from food, people, and pets, and persist throughout life.

• Estimated ratio: 3×1013 human cells vs. 4×1013 bacterial cells.

• The Human Microbiome Project studies the relationship between microbial communities and human health.

Normal vs. Transient Microbiota

• Normal Microbiota (Normal Flora): Permanently colonize the host without causing disease under normal conditions.

• Transient Microbiota: Present temporarily (days to months) and then disappear.

• Distribution and composition are influenced by nutrients, physical/chemical factors, mechanical factors, host defenses, age, diet, geography, hygiene, and lifestyle.

• Normal microbiota contribute to immune system development.

Vaginal vs. Cesarean Birth — Microbiome Differences

Vaginal Birth	Cesarean Birth
Infants encounter diverse microbes; microbiome resembles mother's vaginal flora (e.g., Lactobacillus, Bacteroides); influences immune system development.	Microbiome resembles human skin; Staphylococcus aureus prevalent; higher incidence of type 1 diabetes, asthma, and obesity.

Relationships Between Normal Microbiota and the Host

Normal microbiota interact with the host in various ways, including protective and potentially harmful effects.

• Microbial antagonism (competitive exclusion): Normal microbiota compete with pathogens for nutrients, produce bacteriocins, and alter pH and oxygen levels to inhibit pathogens.

• Disruption of normal flora (e.g., by antibiotics or douching) can lead to infections such as vaginitis or Clostridium difficile colitis.

• Fecal Microbial Transfer (FMT): Restores normal microbiota to treat recurrent C. difficile infections.

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; many pathogens are parasites.

• Opportunistic pathogens: Normal microbiota that can cause disease under certain conditions (e.g., immunosuppression).

Koch's Postulates

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

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 introduced into a healthy, susceptible host.

4. The pathogen must be re-isolated from the experimentally infected host and shown to be the original organism.

• Exceptions: Some pathogens cause multiple diseases, some only infect humans, some cannot be cultured, and different pathogens can cause similar symptoms.

Classifying Infectious Diseases

Diseases are classified based on their signs, symptoms, and modes of transmission.

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

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

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

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

• Contagious Disease: Easily and rapidly spread (e.g., measles).

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

Occurrence of a Disease

Understanding disease occurrence involves measuring incidence and prevalence, and recognizing patterns of disease frequency.

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

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

Term	Definition	Example
Sporadic Disease	Occurs occasionally	Typhoid fever
Endemic Disease	Constantly present in a population	Common cold
Epidemic Disease	Acquired by many in a short time	Influenza
Pandemic Disease	Worldwide epidemic	COVID-19

Severity or Duration of a Disease

Diseases vary in how quickly they develop and how long they last.

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

• Chronic Disease: Develops slowly, lasts a long time (e.g., hepatitis).

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

• Herd Immunity: When most of a population is immune, reducing disease spread.

COVID-19 Severity Examples

Severity	Symptoms
Asymptomatic	No signs or symptoms
Mild	Fever, dry cough, fatigue, sore throat
Moderate	Breathlessness, tachycardia, persistent cough
Severe	Pneumonia, extreme breathlessness, chest pain
Critical	Severe acute respiratory syndrome, inflamed alveoli
Sepsis	Extreme inflammatory response to infection

Extent of Host Involvement

Infections can be classified by their location and spread within the body.

• Local Infection: Confined to a small area (e.g., boils).

• Systemic Infection: Spread throughout the body (e.g., measles).

• Focal Infection: Starts local, spreads systemically (e.g., bacterial endocarditis).

• Sepsis: Toxic inflammatory response from spread of microbes or toxins.

• Bacteremia: Bacteria in the blood.

• Septicemia: Growth of bacteria in the blood.

• Toxemia: Toxins in the blood.

• Viremia: Viruses in the blood.

• Primary Infection: Initial acute infection.

• Secondary Infection: Opportunistic infection after a primary infection.

• Subclinical Infection: No noticeable symptoms (e.g., Hepatitis A).

Predisposing Factors

Various factors can increase susceptibility to disease.

• Nutrition

• Sex

• Genetic inheritance (e.g., sickle cell gene and malaria resistance)

• Climate and environment

• Vaccination status

• Age

• Lifestyle and behaviors

• Compromised host status

Development of Disease

Infectious diseases typically progress through several stages.

1. Incubation Period: Time between infection and first symptoms.

2. Prodromal Period: Short period of mild, nonspecific symptoms.

3. Period of Illness: Disease is most severe.

4. Period of Decline: Symptoms subside.

5. Period of Convalescence: Recovery; body returns to normal.

Reservoirs of Infection

Reservoirs are continual sources of infection.

• Human Reservoirs: People with active disease or carriers (asymptomatic, incubating, convalescent, chronic, or passive).

• Animal Reservoirs: Zoonoses are diseases transmitted from animals to humans (e.g., rabies, Lyme disease).

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

Transmission of Disease

Pathogens can be transmitted through various routes.

• Contact Transmission:

  • Direct Contact: Physical contact between infected and susceptible host.

  • Congenital Transmission: From mother to fetus/newborn.

  • Indirect Contact: Via fomites (nonliving objects).

  • Droplet Transmission: Airborne droplets <1 meter.

• Vehicle Transmission: Transmission by inanimate reservoirs (air, water, food).

• Vector Transmission: Arthropods (fleas, ticks, mosquitoes).

  • Mechanical Transmission: Pathogen carried on vector's body.

  • Biological Transmission: Pathogen reproduces in vector; transmitted via bites or feces.

Healthcare-Associated Infections (HAIs)

HAIs are infections acquired during the course of receiving healthcare treatment.

• Also called nosocomial infections.

• Affect 1 in 31 hospital patients in the U.S.; ~700,000 cases and 70,000 deaths annually.

• Result from microorganisms in the hospital, compromised hosts, and transmission chains.

• Common sources: Clostridium difficile, antimicrobial-resistant organisms.

• Transmission via direct contact (staff-patient, patient-patient) or indirect contact (fomites, ventilation systems).

• Fomites include catheters, needles, respiratory aids, surgical dressings.

Emerging Infectious Diseases

Emerging infectious diseases are those that are new, increasing in incidence, or have the potential to increase soon.

• Most are zoonotic, viral, and vector-borne.

• Contributing factors:

  • Genetic recombination (e.g., E. coli O157:H7, avian influenza H5N1)

  • Evolution of new strains (e.g., Vibrio cholerae O139)

  • Antibiotic/pesticide use (antibiotic resistance)

  • Genetic instability (RNA viruses: antigenic shift/drift)

  • Climate and weather changes (e.g., Hantavirus)

  • Modern transportation (e.g., Zika, West Nile)

  • Ecological changes, war, human settlement (e.g., coccidioidomycosis)

  • Animal control failures (e.g., Lyme disease)

  • Public health failures (e.g., diphtheria)

  • Bioterrorism

Epidemiology

Epidemiology is the study of disease occurrence, distribution, and control in populations.

• Determines etiology, risk factors, and patterns of disease spread.

• Assembles data to predict and control outbreaks.

• Reproductive number (R0): Average number of people infected by one case.

Historical Figures in Epidemiology

Person	Date	Contribution
John Snow	1848–1849	Mapped cholera outbreak in London
Ignaz Semmelweis	1846–1848	Showed handwashing reduced puerperal sepsis
Florence Nightingale	1858	Linked sanitation to reduced typhus incidence

Types of Epidemiology

• Descriptive Epidemiology: Collects and analyzes data on disease occurrence (e.g., Snow's cholera study).

• Analytical Epidemiology: Determines probable cause and risk factors (e.g., Nightingale's work).

• Experimental Epidemiology: Uses controlled experiments to test hypotheses (e.g., Semmelweis's handwashing trials).

Case Reporting and Public Health

• Case reporting helps track disease transmission and outbreaks.

• Physicians must report notifiable diseases to health authorities.

• The CDC collects and publishes data in the Morbidity and Mortality Weekly Report (MMWR).

• Morbidity: Incidence of a specific disease.

• Mortality: Number of deaths from a disease.

• Morbidity Rate: $\text{Morbidity Rate} = \frac{\text{Number of cases}}{\text{Total population}} \times 100,000$

• Mortality Rate: $\text{Mortality Rate} = \frac{\text{Number of deaths}}{\text{Total population}} \times 100,000$

Memory Aids

• Disease Stages: Incubation → Prodromal → Illness → Decline → Convalescence

• Frequency Scale: Sporadic → Endemic → Epidemic → Pandemic

• Symptoms: Subjective (felt by patient) | Signs: Objective (observed/measured)