Principles of Disease and Epidemiology
Principles of Disease and Epidemiology
Pathology, Infection, and Disease (1 of 2)
Learning Objective 14-1: Define pathology, etiology, infection, and infectious disease.
Pathology, Infection, and Disease (2 of 2)
Pathology: The study of disease.
Etiology: The cause of a disease.
Pathogenesis: The manner in which a disease develops.
Infection: Invasion or colonization of the body by pathogens.
Infectious disease: Occurs when an infection results in any change in the state of health.
An infection may exist in the absence of a detectable disease.
Disease may result when a particular type of microorganism locates in a part of the body where it is not normally found.
Human Microbiome (1 of 4)
Learning Objectives:
14-2: Describe how the human microbiome is acquired.
14-3: Compare commensalism, mutualism, and parasitism, and give an example of each.
14-4: Contrast normal microbiota and transient microbiota with opportunistic microorganisms.
Human Microbiome (2 of 4)
Begins to establish in utero.
Placental microbiome: Includes members of the Enterobacteriaceae and Propionibacterium.
Lactobacilli: Colonize neonates' intestine—acquired during a vaginal birth.
More microorganisms are acquired from food, people, and pets.
These microorganisms remain throughout life, increasing or decreasing in number and contributing to health and disease.
Human Microbiome (3 of 4)
Estimated size: Billions of bacterial cells.
Human Microbiome Project: Analyzes relationships between microbial communities on the body and human health.
Normal microbiota (normal flora): Permanently colonize the host and do not cause disease under normal conditions.
Transient microbiota: May be present for days, weeks, or months and then disappear.
Human Microbiome (4 of 4)
Factors Influencing the Normal Microbiota:
Distribution and composition of normal microbiota are determined by many factors including:
Nutrients
Physical and chemical factors
Mechanical factors
Body’s defenses against microbes
Age, diet, nutritional status, geography, personal hygiene, living conditions, occupation, lifestyle.
Normal microbiota play a role in the development of the immune system.
Representative Normal Microbiota by Body Region (1 of 2)
Skin:
Principal Components: Propionibacterium, Staphylococcus, Corynebacterium, Micrococcus, Acinetobacter, Brevibacterium; fungi Candida and Malassezia.
Comments: Most microbes in direct contact don’t become residents due to antimicrobial properties of sweat/oil, keratin barriers, and low pH.
Eyes (Conjunctiva):
Principal Components: Staphylococcus epidermidis, S. aureus, diphtheroids, Propionibacterium, Corynebacterium, streptococci, and Micrococcus.
Comments: Tears and blinking help eliminate or inhibit microbes.
Nose and Throat (Upper Respiratory System):
Principal Components: S. aureus, S. epidermidis, aerobic diphtheroids in the nose; S. epidermidis, S. aureus, diphtheroids, Streptococcus pneumoniae, Haemophilus, and Neisseria in the throat.
Comments: Nasal secretions kill/inhibit many microbes; mucus and ciliary action remove many microbes.
Representative Normal Microbiota by Body Region (2 of 2)
Mouth:
Principal Components: Streptococcus, Lactobacillus, Actinomyces, Bacteroides, Veillonella, Neisseria, Haemophilus, Fusobacterium, Treponema, Staphylococcus, Corynebacterium, and Candida.
Comments: Abundant moisture and warmth support large microbial populations; saliva contains antimicrobial substances.
Large Intestine:
Principal Components: E. coli, Bacteroides, Fusobacterium, Lactobacillus, Enterococcus, Bifidobacterium, Enterobacter, Citrobacter, Proteus, Klebsiella, and Candida.\n - Comments: Contains the largest number of resident microbiota due to moisture and nutrients, preventing growth of pathogens via microbial antagonism.
Urinary and Genital (Reproductive) Systems:
Principal Components: Staphylococcus, Micrococcus, Enterococcus, Lactobacillus, Bacteroides, aerobic diphtheroids, Pseudomonas, Klebsiella, and Proteus in the urethra; lactobacilli, Streptococcus, Clostridium, Candida albicans and Trichomonas vaginalis in the vagina.
Comments: The lower urethra has a resident microbial population; vaginal secretions influence microbial populations due to acidity.
Exploring the Microbiome: Connections Between Birth, Microbiome, and Other Health Conditions
Vaginal birth: Lactobacillus and Bacteroides prevalent.
Cesarean birth: Microbiome resembles human skin, characterized by Staphylococcus aureus.
Relationships Between the Normal Microbiota and the Host (1 of 2)
Microbial antagonism (competitive exclusion): A competition between microbes.
Normal microbiota protect the host by:
Competing for nutrients.
Producing substances harmful to invading microbes.
Affecting pH and available oxygen.
Clostridium difficile: Causes severe intestinal infections when normal microbiota diminish due to antibiotic treatment.
Fecal microbial transfer (FMT) can treat C. difficile infections.
Relationships Between the Normal Microbiota and the Host (2 of 2)
Symbiosis: A relationship between organisms where at least one is dependent on the other, illustrated by the interaction between normal microbiota and the host.
Commensalism: One organism benefits while the other is unaffected.
Mutualism: Both organisms benefit.
Parasitism: One organism benefits at the expense of another; many disease-causing microorganisms are parasites.
Some normal microbiota act as opportunistic pathogens.
The Etiology of Infectious Diseases
Learning Objective 14-5: List Koch’s postulates.
Koch’s Postulates (1 of 2)
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 pathogen from the pure culture must cause disease when inoculated into a healthy, susceptible laboratory animal.
4. The pathogen must be isolated from the inoculated animal and must be shown to be the original organism.
Koch’s Postulates (2 of 2)
Purpose: Koch’s postulates are used to prove the cause of an infectious disease.
Exceptions to Koch’s Postulates:
Some pathogens can cause various disease conditions.
Certain pathogens cause disease exclusively in humans.
Microbes that have never been cultured.
Several different pathogens may produce the same signs and symptoms.
Classifying Infectious Diseases (1 of 3)
Learning Objectives:
14-6: Differentiate a communicable from a noncommunicable disease.
14-7: Categorize diseases according to frequency of occurrence.
14-8: Categorize diseases based on severity.
14-9: Define herd immunity.
Classifying Infectious Diseases (2 of 3)
Symptoms: Subjective changes in body function felt by a patient as a result of disease (not apparent to an observer).
Signs: Objective changes in the body that can be measured or observed due to disease.
Syndrome: A specific group of signs and symptoms accompanying a disease.
Classifying Infectious Diseases (3 of 3)
Communicable disease: Spread from one host to another.
Examples include COVID-19, chickenpox, measles, influenza, genital herpes, tuberculosis.
Contagious diseases: Easily and rapidly spread from one host to another.
Noncommunicable disease: Not spread from one host to another.
Example: Tetanus.
Occurrence of a Disease (1 of 2)
Incidence: Number of people who develop a disease during a particular time period.
Prevalence: Number of people who have a disease at a specified time, incorporating both old and new cases.
Occurrence of a Disease (2 of 2)
Disease classifications:
Sporadic disease: Occurs occasionally.
Endemic disease: Constantly present in a population.
Epidemic disease: Acquired by many people in a given area in a short time.
Pandemic disease: Worldwide epidemic.
Severity or Duration of a Disease (1 of 3)
Duration: Average time individuals have a disease from diagnosis until cured or death.
Acute disease: Symptoms develop rapidly but have a short duration.
Chronic disease: Symptoms develop slowly and persist for a long period.
Subacute disease: Intermediate between acute and chronic.
Latent disease: Causative agent is inactive temporarily but can activate and produce symptoms.
Herd immunity: Immunity in most of a population.
Severity or Duration of a Disease (2 of 3)
Severity: Presence and extent of a disease in the body and its capacity to cause death.
COVID-19 Severity Examples:
Asymptomatic: No signs or symptoms.
Mild: Fever, dry cough, tiredness, muscle pain, sore throat.
Moderate: Breathlessness, tachycardia, persistent cough, higher fever.
Severe: Pneumonia, extreme breathlessness, chest pain, high temperature, bluish lips/face.
Critical: Severe acute respiratory syndrome (SARS), inflamed alveoli requiring ventilation, and possibly sepsis (extreme inflammatory syndrome).
Severity or Duration of a Disease (3 of 3)
Infection fatality ratio (IFR): The total number of deaths attributed to a disease divided by the total number of infected individuals within a specific time.
Lower IFR indicates lower fatalities.
Case fatality ratio (CFR): Proportion of individuals diagnosed who die from that disease within a certain time period.
Extent of Host Involvement (1 of 3)
Local infection: Pathogens limited to a small area of the body.
Systemic infection: An infection that spreads throughout the body via blood and lymph.
Focal infection: Systemic infection that began as a local infection.
Extent of Host Involvement (2 of 3)
Sepsis: Toxic inflammatory condition due to spread of microbes or their toxins from an infection focus.
Bacteremia: Presence of bacteria in the blood.
Septicemia: Growth of bacteria in the blood, generally leading to blood poisoning.
Extent of Host Involvement (3 of 3)
Toxemia: Presence of toxins in the blood.
Viremia: Presence of viruses in the blood.
Primary infection: Acute infection causing initial illness.
Secondary infection: Opportunistic infection following a primary infection.
Subclinical infection: No noticeable signs or symptoms (asymptomatic infection).
Patterns of Disease
Learning Objectives:
14-10: Identify four predisposing factors for disease.
14-11: Sequence the pattern of disease: period of decline, convalescence, illness, prodromal period, incubation period.
Predisposing Factors
Variables that heighten susceptibility to disease or may alter its course, including:
Nutrition, sex, genetic inheritance, climate, environment, vaccination, age, lifestyle, and compromised host.
Development of Disease
Incubation period: Interval between initial infection and when signs/symptoms first appear.
Prodromal period: Brief phase after incubation with early mild nonspecific symptoms.
Period of illness: Disease is most severe at this time.
Period of decline: Signs/symptoms subside.
Period of convalescence: Body returns to a predisease state.
The Spread of Infection (1 of 2)
Learning Objectives:
14-12: Define the reservoir of infection.
14-13: Contrast human, animal, and nonliving reservoirs, providing examples.
14-14: Explain three methods of disease transmission.
Reservoirs of Infection (1 of 2)
Reservoirs: Continual sources of infection.
Human reservoirs:
People with signs/symptoms, or carriers with inapparent infections/latent diseases.
Types of carriers: asymptomatic, incubating, convalescent, chronic, and passive carriers.
Reservoirs of Infection (2 of 2)
Animal reservoirs:
Zoonoses: Diseases primarily in wild and domestic animals that can be transmitted to humans.
Nonliving reservoirs:
Soil and water, foods.
Contact Transmission
Direct contact: Requires close association between the infected and susceptible host.
Congenital transmission: From mother to fetus/newborn at birth.
Indirect contact: Spreads via nonliving objects called fomites.
Droplet transmission: Involves airborne droplets less than 1 meter.
Vehicle Transmission
Definition: Transmission via an inanimate reservoir.
Airborne, waterborne, foodborne.
May involve cross-contamination (transfer of pathogens).
Vectors
Definition: Arthropods (especially fleas, ticks, mosquitoes) that transmit disease.
Methods:
Mechanical transmission: Pathogen carried on the vector’s feet.
Biological transmission: Pathogen reproduces in the vector; transmitted via bites or feces.
Healthcare-Associated Infections (HAIs) (1 of 3)
Learning Objectives:
14-15: Define HAIs and explain their importance.
14-16: Define compromised host.
14-17: List several methods of transmission in hospitals.
14-18: Explain how to prevent HAIs.
Healthcare-Associated Infections (HAIs) (2 of 3)
Definition: Infections acquired while receiving treatment in a healthcare facility; also known as nosocomial infections.
Statistics: Affect 1 in 31 hospital patients in the U.S.; approximately 700,000 infected yearly; over 70,000 deaths.
Healthcare-Associated Infections (HAIs) (3 of 3)
Causes:
Microorganisms in the hospital environment, weakened host state, chain of transmission.
Compromised host: An individual whose resistance to infection is impaired by disease or therapy.
Microorganisms in the Hospital
Importance: Hospitals serve as major reservoirs for pathogens.
Opportunistic normal microbiota: High antimicrobial resistance among HAIs; Clostridium difficile is a leading cause of HAIs.
Control of Healthcare-Associated Infections (1 of 3)
Universal precautions: Designed to reduce transmission of microbes in healthcare settings.
Protection for patients/residents/staff/visitors.
Standard precautions:
Minimum practices applied to every person, every time:
Hand hygiene, PPE, respiratory hygiene, environmental cleaning, safe infection practices.
Control of Healthcare-Associated Infections (2 of 3)
Transmission-based precautions: Supplement standard precautions for known or suspected infections that are highly transmissible, categorized into:
Contact precautions
Droplet precautions
Airborne precautions
Control of Healthcare-Associated Infections (3 of 3)
Prevention methods:
Handwashing, disinfecting tubs, cleaning instruments, using disposable equipment.
Improve patients’ resistance to infection; avoid unnecessary antibiotics/immunosuppressive drugs.
Emerging Infectious Diseases (1 of 5)
Learning Objective:
14-19: List probable reasons for emerging infectious diseases and examples.
Emerging Infectious Diseases (2 of 5)
Definition: New or changing diseases, increasing in incidence, or with the potential to increase.
Statistics: 75% of these diseases are zoonotic and primarily of viral origin, likely vector-borne.
Emerging Infectious Diseases (3 of 5)
Contributing factors include:
Genetic recombination (e.g. Escherichia coli O157:H7, avian influenza H5N1).
Evolution of existing organisms (e.g. Vibrio cholerae O139).
Antibiotic/pesticide use leading to resistant strains.
Genetic instability characteristic of some microbes (e.g. RNA viruses).
Emerging Infectious Diseases (4 of 5)
Contributing factors continued:
Changes in global climate (e.g. Hantavirus).
Modern transportation promoting spread (e.g. Zika virus, Chikungunya).
Invasive vector species (e.g. Aedes aegypti, A. albopictus).
Emerging Infectious Diseases (5 of 5)
Public health failures:
Examples include diphtheria and bioterrorism.
Epidemiology (1 of 2)
Learning Objectives:
14-20: Define epidemiology and describe three types of investigations.
14-21: Identify the function of the CDC.
14-22: Define morbidity, mortality, and notifiable infectious diseases.
Epidemiology (2 of 2)
Definition: The study of where and when diseases occur and their transmission in populations.
Evolution of Epidemiology
Key Figures:
John Snow (1848-1849): Mapped cholera in London.
Ignaz Semmelweis (1846-1848): Handwashing decreased puerperal sepsis incidence.
Florence Nightingale (1858): Improved sanitation decreased epidemic typhus incidence.
Roles of Epidemiology
Key Functions:
Determine disease etiology; identify factors/patterns related to disease spread.
Assemble incidence data/graphs; predict infection spread probability (reproductive number).
Explore controlling methods for diseases.
Case Reporting
Importance: Allows researchers to establish chains of transmission, reports specified diseases to health officials, and provides early warning of possible outbreaks.
Nationally Notifiable Infectious Diseases, 2022 (Part 1 of 3)
Examples Include:
Anthrax
Arboviral diseases: neuroinvasive and non-neuroinvasive
Babesiosis
Botulism
Brucellosis
Campylobacteriosis
Candida auris
Carbapenem-resistant Enterobacteriaceae
Chancroid
Chlamydia trachomatis infection
Cholera
Coccidioidomycosis
Congenital syphilis
Coronavirus disease 2019
Cryptosporidiosis
Cyclosporiasis
Dengue virus infections
Diphtheria
Ehrlichiosis and anaplasmosis
Giardiasis
Gonorrhea
Haemophilus influenza invasive disease
Hansen’s disease (leprosy)
Nationally Notifiable Infectious Diseases, 2022 (Part 2 of 3)
Examples Include:
Hantavirus pulmonary syndrome
Hemolytic uremic syndrome
Hepatitis A, B, and C
HIV infection
Influenza-associated pediatric mortality
Invasive pneumococcal disease
Legionellosis
Leptospirosis
Listeriosis
Lyme disease
Malaria
Measles
Meningococcal disease
Mumps
Novel influenza A virus infections
Pertussis (whooping cough)
Plague
Poliovirus infection (both paralytic and non-paralytic)
Psittacosis
Q fever
Rabies (animal or human)
Rubella and congenital rubella syndrome
Salmonellosis
Nationally Notifiable Infectious Diseases, 2022 (Part 3 of 3)
Examples Include:
Severe acute respiratory syndrome associated with coronavirus disease
Shiga toxin-producing E. coli
Shigellosis
Smallpox
Spotted fever rickettsiosis
Streptococcal toxic shock syndrome
Syphilis
Tetanus
Toxic shock syndrome (nonstreptococcal)
Trichinellosis
Tuberculosis
Tularemia
Typhoid fever (S. enterica Typhi)
Vancomycin-intermediate S. aureus (VISA) and Vancomycin-resistant S. aureus (VRSA)
Varicella
Vibriosis
Viral hemorrhagic fever
Yellow fever
Zika virus disease and its congenital infection.
The Centers for Disease Control and Prevention (CDC) (1 of 2)
Role: Collects/analyzes epidemiological information in the U.S.; publishes the Morbidity and Mortality Weekly Report (MMWR).
Definitions:
Morbidity: Incidence of a specific notifiable disease.
Mortality: Deaths from notifiable diseases.
Articles provide outbreak reports, case histories, and disease summaries.
The Centers for Disease Control and Prevention (CDC) (2 of 2)
Notifiable Infectious Diseases: Diseases that must be reported by physicians.
Morbidity rate: Number of people affected per total population over time.
Mortality rate: Number of deaths from a disease per total population over time.
Check Your Understanding
What are the three objectives of pathology?
How do normal microbiota differ from transient microbiota?
Give several examples of microbial antagonism.
How can opportunistic pathogens cause infections?
Explain some exceptions to Koch’s postulates.
What is a predisposing factor?
Does Clostridium perfringens cause a communicable disease?
Distinguish the incidence from the prevalence of a disease.
List two examples of acute and chronic diseases.
How does herd immunity develop?
Why are carriers important reservoirs of infection?
How are zoonoses transmitted to humans?
Give an example of contact transmission, vehicle transmission, mechanical transmission, and biological transmission.
What interacting factors lead to healthcare-associated infections?
What is a compromised host?
How are HAIs primarily transmitted, and how can they be prevented?
Give several examples of emerging infectious diseases.
What is the CDC’s function?
In a recent year, the morbidity of West Nile encephalitis was 5674 and the mortality was 286. The morbidity of listeriosis was 121; and the mortality was 13. Which disease is more likely to be fatal?
After learning that 40 hospital employees developed nausea and vomiting, the hospital infection control officer determined that 39 ill people ate green beans compared to 34 healthy people in the hospital cafeteria the same day. What type of epidemiology is this?