Principles of Disease and Epidemiology – Study Notes

Principles of Disease and Epidemiology

Pathology, Infection, and Disease

This section introduces the foundational concepts of disease in microbiology, focusing on the study of disease, its causes, and how infections develop.

• 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 manner in which a disease develops within the host.

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

• Infectious Disease: A disease in which an infection results in a change from a healthy state.

• An infection may exist without causing detectable disease, and disease can result when microbes colonize areas where they are not normally found.

The Human Microbiome

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

• Acquisition: Begins in utero, with further colonization during birth (especially vaginal birth) and from the environment, food, and contact with others.

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

• Transient Microbiota: Microbes present temporarily (days to months) before disappearing.

• Factors Influencing Microbiota: Nutrients, physical and chemical factors, mechanical factors, host defenses, age, diet, hygiene, and lifestyle.

• Normal microbiota contribute to immune system development and overall health.

Relationships Between Microbiota and the Host

Microorganisms interact with the host in various ways, ranging from beneficial to harmful.

• Microbial Antagonism (Competitive Exclusion): Normal microbiota protect the host by competing with pathogens for nutrients, producing inhibitory substances, and altering environmental conditions (e.g., pH, oxygen).

• Symbiosis: A close relationship between two organisms, classified as:

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

  • Mutualism: Both organisms benefit (e.g., E. coli in the large intestine).

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

• Some normal microbiota are opportunistic pathogens, causing disease when the host's defenses are compromised.

Koch’s Postulates and the Etiology of Infectious Diseases

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

• The same pathogen must be present in every case of the disease.

• The pathogen must be isolated from the diseased host and grown in pure culture.

• The cultured pathogen must cause the disease when introduced into a healthy, susceptible host.

• The pathogen must be re-isolated from the experimentally infected host and identified as the original organism.

• Exceptions: Some pathogens cannot be cultured, cause multiple diseases, or only infect humans.

Classifying Infectious Diseases

Diseases are classified based on their communicability, frequency, and severity.

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

• Signs: Objective changes measurable or observable by others (e.g., fever, rash).

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

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

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

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

Occurrence of Disease

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

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

• Sporadic Disease: Occurs occasionally.

• Endemic Disease: Constantly present in a population.

• Epidemic Disease: Many people acquire in a short time.

• Pandemic Disease: Worldwide epidemic.

Severity or Duration of Disease

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

• Chronic Disease: Develops slowly, lasts long (e.g., tuberculosis).

• Subacute Disease: Intermediate between acute and chronic.

• Latent Disease: Pathogen inactive for a time, then activates (e.g., herpes simplex).

• Herd Immunity: Immunity in most of a population, reducing disease spread.

Infection Fatality Ratio (IFR): $\text{IFR} = \frac{\text{deaths from the disease}}{\text{total infections}}$

Case Fatality Ratio (CFR): $\text{CFR} = \frac{\text{deaths from the disease}}{\text{confirmed cases}}$

Extent of Host Involvement

• Local Infection: Pathogens limited to a small area.

• Systemic Infection: Spread throughout the body via blood or lymph.

• Focal Infection: Systemic infection that began as a local infection.

• Sepsis: Toxic inflammatory condition 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 signs or symptoms.

Patterns of Disease

The development of disease follows a predictable sequence of stages.

• Predisposing Factors: Variables increasing susceptibility (e.g., nutrition, age, genetics, environment, lifestyle).

• Stages of Disease:

  • Incubation Period: Time between infection and first symptoms.

  • Prodromal Period: Early, mild symptoms.

  • Period of Illness: Most severe signs and symptoms.

  • Period of Decline: Signs and symptoms subside.

  • Period of Convalescence: Recovery; return to prediseased state.

The Spread of Infection

Understanding reservoirs and modes of transmission is essential for controlling infectious diseases.

• Reservoirs of Infection: Continual sources of infection.

  • Human Reservoirs: Symptomatic individuals or carriers (asymptomatic, incubating, convalescent, chronic, passive).

  • Animal Reservoirs: Zoonoses are diseases transmitted from animals to humans.

  • Nonliving Reservoirs: Soil, water, and food.

Modes of Transmission

• Contact Transmission:

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

  • Indirect Contact: Via fomites (nonliving objects).

  • Droplet Transmission: Airborne droplets less than 1 meter.

  • Congenital Transmission: From mother to fetus or newborn.

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

• Vector Transmission: Arthropods (e.g., fleas, ticks, mosquitoes) transmit disease by:

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

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

Healthcare-Associated Infections (HAIs)

HAIs, also known as nosocomial infections, are acquired during the course of receiving treatment in a healthcare facility.

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

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

• Compromised Host: Individual with impaired resistance due to disease, therapy, or burns.

• Common transmission routes: direct contact (staff to patient, patient to patient), indirect contact (fomites, ventilation systems), invasive devices (catheters, needles).

• Control Measures: Universal and standard precautions (hand hygiene, PPE, disinfection), transmission-based precautions (contact, droplet, airborne), and infection control committees.

Emerging Infectious Diseases

Emerging infectious diseases are new, changing, or increasing in incidence, often zoonotic and viral in origin.

• Contributing Factors:

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

  • Evolution of existing organisms (e.g., Vibrio cholerae O139)

  • Widespread use of antibiotics/pesticides (antibiotic resistance)

  • Changes in climate and weather (e.g., Hantavirus)

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

  • Ecological changes, war, expanding settlements (e.g., coccidioidomycosis)

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

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

  • Bioterrorism

Epidemiology

Epidemiology is the study of where and when diseases occur and how they are transmitted in populations.

• Roles: Determine etiology, identify patterns, assemble data, predict spread, and explore control methods.

• Types of Epidemiologic Investigations:

  • Descriptive Epidemiology: Collection and analysis of data (e.g., John Snow and cholera).

  • Analytical Epidemiology: Determines probable cause/risk factors (e.g., Florence Nightingale).

  • Experimental Epidemiology: Hypothesis-driven, controlled experiments (e.g., Semmelweis).

• Case Reporting: Health workers report specified diseases to authorities, enabling outbreak detection and control.

• Notifiable Infectious Diseases: Diseases that must be reported to public health officials (e.g., anthrax, measles, tuberculosis).

• CDC (Centers for Disease Control and Prevention): Collects/analyzes epidemiological data, publishes the Morbidity and Mortality Weekly Report (MMWR).

• Morbidity: Incidence of a specific notifiable disease.

• Mortality: Deaths from notifiable diseases.

• Morbidity Rate: $\text{Morbidity Rate} = \frac{\text{number of people affected}}{\text{total population in a given time period}}$

• Mortality Rate: $\text{Mortality Rate} = \frac{\text{number of deaths}}{\text{total population in a given time period}}$

Additional info: These notes synthesize and expand upon the provided lecture slides and textbook images, ensuring a comprehensive, exam-ready overview of Chapter 14: Principles of Disease and Epidemiology.