Principles of Disease and Epidemiology: Microbiology Study Notes

Principles of Disease and Epidemiology

Introduction

This chapter explores the fundamental concepts of disease, epidemiology, and the interactions between microbes and their human hosts. Understanding these principles is essential for recognizing how infectious diseases develop, spread, and can be controlled within populations.

Pathology, Infection, and Disease

Key Definitions

• Pathology: The study of disease (including its causes, development, and effects on the body)/

• Etiology: The cause of a disease.

• Pathogenesis: The development and progression of disease.

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

• Disease: An abnormal state in which the body is not functioning normally.

The Human Microbiome

Acquisition and Types of Microbiota

• The human microbiome may start getting established before birth, and then it keeps developing after birth through exposure to food, people, pets, etc.

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

• Transient microbiota are present temporarily and may persist for days, weeks, or months.

The Human Microbiome Project analyzes the relationships between microbial communities and human health.

• The distribution and composition of normal microbiota are influenced by factors such as nutrients, physical and chemical conditions, host defenses, and mechanical factors.

Representative Normal Microbiota by Body Region

Region	Principal Components	Comments
Skin	Propionibacterium, Staphylococcus, Corynebacterium, Micrococcus, Acinetobacter, Brevibacterium, Candida (fungus), Malassezia (fungus)	Most microbes in direct contact with skin do not become residents due to secretions and shedding. Skin is a relatively inhospitable environment.
Eyes (conjunctiva)	Staphylococcus epidermidis, S. aureus, Corynebacterium, Propionibacterium, Micrococcus, Streptococcus, Haemophilus	Tears and blinking eliminate some microbes; others are resistant to these defenses.
Nose and Throat (Upper Respiratory System)	Staphylococcus aureus, S. epidermidis, aerobic diphtheroids, Streptococcus pneumoniae, Haemophilus, Neisseria	Microbes are inhaled and trapped in mucus; some are eliminated by ciliary action, others become residents.
Mouth	Streptococcus, Lactobacillus, Actinomyces, Bacteroides, Veillonella, Neisseria, Corynebacterium, Candida (fungus)	Chewing, saliva, and tongue movements dislodge microbes; some attach to teeth and gums.
Large Intestine	Escherichia coli, Enterobacter, Proteus, Klebsiella, Lactobacillus, Clostridium, Bacteroides, Fusobacterium, Enterococcus, Candida (fungus)	Largest microbial population; mucus and shedding cells eliminate many, but the environment supports growth of anaerobes and facultative anaerobes.
Urinary and Reproductive Systems	Staphylococcus, Micrococcus, Enterococcus, Lactobacillus, Corynebacterium, Streptococcus, Candida (fungus)	Urine flow and vaginal secretions remove microbes; the environment supports growth of certain bacteria and fungi.

Birth and Microbiome Development

• Vaginal birth: Infants are colonized predominantly by Lactobacillus

• Caesarean birth: The microbiome resembles that of human skin, with Staphylococcus aureus being common.

Relationships Between Normal Microbiota and the Host

• Microbial antagonism (competitive exclusion): Normal microbiota protect the host by competing for nutrients, producing substances harmful to invading microbes, and affecting pH and oxygen availability.

• Symbiosis: The relationship between normal microbiota and the host, which can be:

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

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

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

• Some normal microbiota can become opportunistic pathogens if the host's defenses are compromised.

The Etiology of Infectious Diseases

Koch’s Postulates

• 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 pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible laboratory animal.

• The pathogen must be re-isolated from the inoculated animal and shown to be the original organism.

Koch’s postulates are used to prove the cause of an infectious disease, but there are exceptions:

• Some pathogens cause multiple disease conditions.

• Some pathogens only infect humans.

• Some microbes cannot be cultured in the laboratory.

Classifying Infectious Diseases

Symptoms, Signs, and Syndromes

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

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

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

Communicable vs. Noncommunicable Diseases

• Communicable disease: Spread 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 Disease

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

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

• Sporadic disease: Occurs occasionally (e.g., typhoid fever).

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

• Epidemic disease: Many people acquire the disease in a short time (e.g., influenza outbreak).

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

Severity or Duration of Disease

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

• Chronic disease: Develops slowly, lasts a long time (e.g., tuberculosis).

• Subacute disease: Intermediate between acute and chronic.

• Latent disease: Causative agent is inactive for a time, then activates (e.g., herpes simplex).

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

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Patterns of Disease

Predisposing Factors

• Gender

• Inherited traits (e.g., sickle cell gene)

• Climate and weather

• Lack of vaccination

• Fatigue

• Age

• Lifestyle

• Nutrition

• Chemotherapy

Development of Disease

• Incubation period: Interval between initial infection and first symptoms.

• Prodromal period: Short period of early, mild symptoms.

• Period of illness: Disease is most severe.

• Period of decline: Signs and symptoms subside.

• Period of convalescence: Body returns to prediseased state.

The Spread of Infection

Reservoirs of Infection

• Human reservoirs: Carriers may have inapparent infections or latent diseases.

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

• Nonliving reservoirs: Soil and water can harbor pathogens.

Transmission of Disease

• Contact transmission:

  • Direct contact: Close association between infected and susceptible host.

  • Indirect contact: Spread by fomites (nonliving objects).

  • Droplet transmission: Airborne droplets less than 1 meter.

  • Congenital transmission: From mother to fetus or newborn at birth.

• 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)

Overview

• HAIs are acquired while receiving treatment in a healthcare facility (nosocomial infections).

• Affect 1 in 25 hospital patients; significant cause of morbidity and mortality.

• Result from microorganisms in the hospital, weakened host status, and chain of transmission.

• Compromised host: Individual with impaired resistance to infection.

Control of HAIs

• Universal and standard precautions (handwashing, cleaning instruments, using disposables).

• Transmission-based precautions for known or suspected infections (contact, droplet, airborne).

• Infection control committees monitor and implement prevention strategies.

Emerging Infectious Diseases

Contributing Factors

• Genetic recombination (e.g., Escherichia coli O157, avian influenza H5N1)

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

• Widespread use of antibiotics and pesticides (antibiotic-resistant strains)

• Changes in weather patterns (e.g., Hantavirus)

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

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

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

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

• Bioterrorism

Epidemiology

Definition and Types of Investigations

• Epidemiology: The study of where and when diseases occur and how they are transmitted in populations.

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

• Analytical epidemiology: Analyzes a disease to determine its probable cause (e.g., Florence Nightingale and typhus).

• Experimental epidemiology: Involves hypothesis and controlled experiments (e.g., Ignaz Semmelweis and puerperal sepsis).

Centers for Disease Control and Prevention (CDC)

• Collects and analyzes epidemiological information in the United States.

• Publishes the Morbidity and Mortality Weekly Report (MMWR).

• Morbidity: Incidence of a specific notifiable disease.

• Mortality: Deaths from notifiable diseases.

• Morbidity rate: Number of people affected in relation to the total population in a given time period.

• Mortality rate: Number of deaths from a disease in relation to the population in a given time.

Summary Table: Key Terms and Concepts

Term	Definition
Pathology	Study of disease
Etiology	Cause of a disease
Infection	Invasion or colonization by pathogens
Disease	Abnormal state of body function
Incidence	New cases in a time period
Prevalence	Total cases at a specific time
Herd Immunity	Immunity in most of a population