Principles of Disease and Epidemiology: Microbiology Study Guide

Principles of Disease and Epidemiology

Pathology, Infection, and Disease

Pathology is the scientific study of disease, focusing on its causes, development, and effects on the body. Understanding these concepts is fundamental for microbiology students.

• Pathology: The study of disease, including its origin and effects.

• Etiology: The cause of a disease.

• Pathogenesis: The process by which disease develops.

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

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

The Human Microbiome

The human microbiome consists of the collection of microorganisms that inhabit various regions of the human body. These microbes play essential roles in health and disease.

• Acquisition: Begins in utero and continues through exposure to food, people, and pets.

• Normal microbiota: Permanently colonize the host and typically do not cause disease.

• Transient microbiota: Present temporarily (days, weeks, or months).

• Factors affecting microbiota: Nutrients, physical and chemical factors, host defenses, and mechanical factors.

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

Representative Normal Microbiota by Body Region

Different regions of the body host distinct microbial populations, influenced by environmental and physiological factors.

Region	Principal Components	Comments
Skin	Propionibacterium, Staphylococcus, Corynebacterium, Micrococcus, Acinetobacter, Brevibacterium; Candida, Malassezia	Antimicrobial secretions, low pH, low moisture
Eyes	Staphylococcus epidermidis, S. aureus, diphtheroids, Propionibacterium, Corynebacterium, streptococci, Micrococcus	Tears and blinking remove microbes
Nose and Throat	Staphylococcus aureus, S. epidermidis, diphtheroids, Streptococcus pneumoniae, Haemophilus, Neisseria	Microbial antagonism, nasal secretions, mucus
Mouth	Streptococcus, Lactobacillus, Actinomyces, Bacteroides, Veillonella, Neisseria, Haemophilus, Fusobacterium, Treponema, Staphylococcus, Corynebacterium, Candida	Moisture, food, saliva, mechanical removal
Large Intestine	Escherichia coli, Bacteroides, Fusobacterium, Lactobacillus, Enterococcus, Bifidobacterium, Enterobacter, Citrobacter, Proteus, Klebsiella, Candida	Largest numbers, mucus, antimicrobial chemicals
Urinary & Reproductive Systems	Staphylococcus, Micrococcus, Enterococcus, Lactobacillus, Bacteroides, diphtheroids, Pseudomonas, Klebsiella, Proteus, lactobacilli, Streptococcus, Clostridium, Candida albicans, Trichomonas vaginalis	Acid-tolerant, mucus, urine flow, pH

Microbiome and Birth

The method of birth influences the initial microbiome composition in newborns.

• Vaginal birth: Microbiome dominated by Lactobacillus and Bacteroides.

• Cesarean birth: Microbiome resembles human skin, with Staphylococcus aureus prevalent.

Microbial Antagonism and Symbiosis

Normal microbiota interact with the host and invading microbes through various relationships.

• Microbial antagonism: Competition between microbes, protecting the host by competing for nutrients, producing harmful substances, and affecting pH and oxygen.

• Symbiosis: Relationship between microbiota and host.

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

  • Mutualism: Both organisms benefit.

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

• Opportunistic pathogens: Normal microbiota that can cause disease under certain conditions.

The Etiology of Infectious Diseases

Koch’s Postulates

Koch’s postulates are a set of criteria used to establish the 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 pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible laboratory animal.

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

Exceptions: Some pathogens cause multiple diseases, some only infect humans, and some cannot be cultured.

Classifying Infectious Diseases

Symptoms, Signs, and Syndromes

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

• Symptoms: Subjective changes felt by the patient.

• Signs: Objective changes measurable or observable.

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

Types of Diseases

• Communicable disease: Spread from one host to another.

• Contagious disease: Easily and rapidly spread.

• Noncommunicable disease: Not spread from host to host.

Occurrence of Disease

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

• Prevalence: Total number of cases at a specified time, including old and new cases.

Frequency and Severity of Disease

• Sporadic disease: Occurs occasionally.

• Endemic disease: Constantly present in a population.

• Epidemic disease: Acquired by many people in a short time.

• Pandemic disease: Worldwide epidemic.

• Acute disease: Rapid onset, short duration.

• Chronic disease: Slow development, long duration.

• Subacute disease: Intermediate between acute and chronic.

• Latent disease: Inactive for a time, then activates.

• Herd immunity: Immunity in most of a population.

Extent of Host Involvement

Types of Infection

• Local infection: Limited to a small area.

• Systemic infection: Throughout the body.

• 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 disease: No noticeable signs or symptoms.

Patterns of Disease

Predisposing Factors

Predisposing factors increase susceptibility to disease.

• Gender

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

• Climate and weather

• Lack of vaccination

• Fatigue

• Age

• Lifestyle

• Nutrition

• Chemotherapy

Stages of Disease

The progression of disease follows distinct stages, each with characteristic signs and symptoms.

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

• Prodromal period: Early, mild symptoms.

• Period of illness: Most severe symptoms.

• Period of decline: Symptoms subside.

• Period of convalescence: Return to prediseased state.

The Spread of Infection

Reservoirs of Infection

Reservoirs are continual sources of infection.

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

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

• Nonliving reservoirs: Soil and water.

Transmission of Disease

• Contact transmission: Direct (close association), indirect (fomite), droplet (airborne droplets <1 meter), congenital (mother to fetus).

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

• Vector transmission: Arthropods transmit disease mechanically (pathogen on feet) or biologically (pathogen reproduces in vector).

Healthcare-Associated Infections (HAIs)

Definition and Importance

HAIs, also known as nosocomial infections, are acquired while receiving treatment in healthcare facilities. They are a major concern due to their frequency and impact.

• Affect 1 in 25 hospital patients.

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

• Compromised host: Resistance to infection is impaired.

Common Microorganisms in HAIs

Microorganism	Most Common Infection Type	Percentage of Total Infections	Percentage Resistant to Antibiotics
Coagulase-negative staphylococci	Bloodstream	11%	Not reported
Staphylococcus aureus	Surgical wound	16%	55%
Clostridium difficile	Diarrhea after abdominal surgery	15%	Not reported
Enterococcus spp.	Bloodstream	14%	83%
Candida spp. (fungus)	Urinary tract infections	9%	Not reported
Escherichia coli	Urinary tract infections	12%	20%
Pseudomonas aeruginosa	Urinary tract and pneumonia	8%	10%
Klebsiella pneumoniae	All sites	8%	29%
Enterobacter spp.	All sites	5%	38%
Acinetobacter baumannii	All sites	2%	68%

Control and Prevention of HAIs

• Universal precautions: Standard and transmission-based precautions.

• Handwashing, disinfecting, cleaning instruments, using disposable items.

• Infection control committees.

Emerging Infectious Diseases

Definition and Contributing Factors

Emerging infectious diseases are new, increasing in incidence, or likely to increase. Most are zoonotic, viral, and vector-borne.

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

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

• Antibiotic resistance: Due to widespread use

• Weather changes: e.g., hantavirus

• Modern transportation: e.g., Zika virus

• Insect vectors: e.g., Aedes aegypti

• Ecological disaster, war, human settlement: e.g., coccidioidomycosis

• Animal control measures: e.g., Lyme disease

• Public health failure: e.g., diphtheria

• Bioterrorism

Epidemiology

Definition and Types of Investigation

Epidemiology is the study of where and when diseases occur and how they are transmitted in populations. Epidemiologists determine etiology, identify spread factors, develop control methods, and assemble data.

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

• Analytical epidemiology: Analyzes probable cause (e.g., Florence Nightingale).

• Experimental epidemiology: Hypothesis and controlled experiments (e.g., Ignaz Semmelweis).

The Centers for Disease Control and Prevention (CDC)

• Collects and analyzes epidemiological information in the U.S.

• Publishes Morbidity and Mortality Weekly Report (MMWR).

• Morbidity: Incidence of a specific notifiable disease.

• Mortality: Deaths from notifiable diseases.

• Notifiable infectious diseases: Diseases physicians are required to report.

• Morbidity rate: Number of people affected relative to population.

• Mortality rate: Number of deaths relative to population.

Nationally Notifiable Infectious Diseases (2017)

Examples include anthrax, arboviral diseases, babesiosis, botulism, brucellosis, campylobacteriosis, chlamydia, cholera, dengue, diphtheria, gonorrhea, hepatitis, HIV, influenza, Lyme disease, malaria, measles, mumps, pertussis, plague, rabies, salmonellosis, syphilis, tuberculosis, typhoid fever, varicella, yellow fever, Zika virus disease, and more.