Chapter 14-Principles of Disease and Epidemiology

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Principles of Disease and Epidemiology

Key Definitions and Concepts

This section introduces foundational terms and concepts in the study of disease and epidemiology, essential for understanding how diseases develop and spread.

Pathology: The study of disease, focusing on structural and functional changes in the body.
Etiology: The study of the cause of a disease.
Pathogenesis: The development and progression of disease.
Pathogenicity: The ability of a microorganism to cause disease by overcoming host defenses.
Virulence: The degree or extent of pathogenicity.
Infection: Colonization or invasion of the body by pathogens.
Disease: An abnormal state in which the body is not functionally normal.

Normal Microbiota and the Host

Types and Roles of Microbiota

Normal microbiota are microorganisms that inhabit various regions of the human body, playing crucial roles in health and disease.

Transient microbiota: Present for days, weeks, or months.
Normal microbiota: Permanently colonize the host and do not cause disease under normal conditions.
Symbiosis: Relationship between normal microbiota and the host, classified as:
- Commercialism: One organism benefits, the other is unaffected.
- Mutualism: Both organisms benefit.
- Parasitism: One organism benefits at the expense of the other.
Opportunistic pathogens: Some normal microbiota can become pathogenic if host immunity is compromised.
Microbial antagonism (competitive exclusion): Competition between microbes, helping protect the host.
Probiotics: Live microbes applied to or ingested into the body to exert beneficial effects.

Additional info: The human body carries approximately bacteria, about ten times more than human cells.

Factors Affecting Microbiota Distribution

Nutrients

Physical and chemical factors
Host defenses
Mechanical factors

Protection by Normal Microbiota

Competing for nutrients
Occupying niches that pathogens might occupy
Producing acids
Producing bacteriocins (proteinaceous toxins that inhibit growth of similar or closely related bacterial strains)

Representative Normal Microbiota by Body Region

Distribution and Comments

Region	Comments
Skin	Most microbes in direct contact with skin do not become residents due to antimicrobial properties of sweat and oil gland secretions. Keratin acts as a resistant barrier; low pH inhibits many microbes. Low moisture content further limits microbial colonization.
Eyes (Conjunctiva)	Contains similar microbiota as skin. Tears and blinking eliminate or inhibit colonization by microbes.
Nose and Throat (Upper Respiratory System)	Some normal microbiota are potential pathogens, but microbial antagonism reduces disease risk. Nasal secretions and mucus, along with ciliary action, remove or kill many microbes.
Mouth	Moisture, warmth, and food support large, diverse microbial populations. Biting, chewing, tongue movements, and salivary flow dislodge microbes; saliva contains antimicrobial substances.
Large Intestine	Largest numbers of resident microbiota due to moisture and nutrients. Mucus and periodic shedding prevent attachment; mucosa produces antimicrobial chemicals. Diarrhea flushes out some microbiota.
Urinary and Reproductive Systems	Lower urethra and vagina have resident, acid-tolerant populations. Mucus and shedding prevent attachment; urine flow and pH are antimicrobial. Cilia and mucus expel microbes from cervix; vaginal acidity inhibits or kills microbes.

Koch's Postulates

Establishing Disease Etiology

Koch's postulates are a set of criteria used to prove that a specific microorganism causes a particular 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 animal.
The pathogen must be isolated from the inoculated animal and shown to be the original organism.

Example: Koch's postulates were used to identify the causative agent of anthrax (Bacillus anthracis).

Classifying Infectious Diseases

Symptoms, Signs, and Syndromes

Symptom: Subjective change in body function felt by the patient (e.g., pain, malaise).
Sign: Objective change that can be measured or observed (e.g., fever, rash).
Syndrome: Specific group of signs and symptoms that accompany a disease.

Types of Diseases

Communicable disease: Can be spread from one host to another (e.g., tuberculosis).
Contagious disease: Easily and rapidly spread by direct contact or body fluids (e.g., chickenpox).
Noncommunicable disease: Not transmitted from one host to another.

Occurrence of Disease

Incidence: Fraction of a population that contracts a disease during a specific time period.
Prevalence: Fraction of a population having a specific disease at a given time (includes old and new cases).
Sporadic disease: Occurs only occasionally.
Endemic disease: Constantly present in a population.
Epidemic disease: Acquired by many hosts in a given area in a short time.
Pandemic disease: Worldwide epidemic.
Herd immunity: Immunity in most of a population.

Severity or Duration of Disease

Acute disease: Symptoms develop rapidly, last a short time (e.g., influenza).
Chronic disease: Develops slowly, may be continual or recurrent (e.g., tuberculosis, hepatitis B).
Subacute disease: Symptoms between acute and chronic.
Latent disease: Period of no symptoms when the causative agent is inactive, then becomes active (e.g., shingles).

Extent of Host Involvement

Local infection: Pathogens limited to a small area (e.g., boils, abscesses).
Systemic infection: Microbes or their products spread throughout the body via blood or lymph (e.g., measles).
Focal infection: Systemic infection that began as a local infection and spread to other specific areas (e.g., infection from teeth).

Bacteremia: Bacteria in the blood.
Septicemia: Growth of bacteria in the blood causing systemic infection (blood poisoning).
Toxemia: Toxins in the blood (e.g., tetanus).
Viremia: Viruses in the blood.
Primary infection: Acute infection causing initial illness.
Secondary infection: Opportunistic infection after a primary infection.
Subclinical infection: No noticeable signs or symptoms (inapparent infection).

Predisposing Factors

Factors Increasing Susceptibility to Disease

Gender
Inherited traits (e.g., sickle cell gene)
Climate and weather
Fatigue
Age
Lifestyle
Nutrition
Chemotherapy

The Stages of Disease

Progression of Infectious Disease

Once a microbe overcomes host defenses, the development of disease follows a typical sequence:

Incubation Period: Time between initial infection and first appearance of signs or symptoms. Depends on microorganism type, virulence, number of infecting organisms, and host resistance.
Prodromal Period: Early, mild symptoms (e.g., general aches, malaise).
Period of Illness: Disease is most severe; white blood cell count may change. If not overcome, patient may die.
Period of Decline: Signs and symptoms subside; patient vulnerable to secondary infections.
Period of Convalescence: Recovery; body returns to pre-disease state.

Additional info: The stages can be visualized as a curve showing the number of microbes over time, with the period of illness at the peak.

The Spread of Infection

Reservoirs of Infection

Reservoirs are continual sources of infection.

Human: AIDS, gonorrhea; carriers may have inapparent or latent infections.
Animal: Rabies, Lyme disease; zoonoses are diseases primarily in animals but transmissible to humans.
Nonliving: Soil, water (e.g., botulism, tetanus).

Transmission of Disease

Contact Transmission:
- Direct: Close association between infected and susceptible host.
- Indirect: Spread by fomites (inanimate objects such as tissues, towels, bedding, diapers, drinking cups).
- Droplet: Transmission via airborne droplets.
Vehicle Transmission: Transmission by an inanimate reservoir (water, food, air).
Vector Transmission: Arthropods (fleas, ticks, mosquitoes) transmit disease by:
- Mechanical (passive): Pathogen carried on feet.
- Biological (active): Pathogen reproduces in vector.

Healthcare-Associated Infections (HAIs)

Nosocomial Infections

HAIs are infections acquired while receiving treatment in a healthcare facility.

Affect 1 in 25 hospital patients (2 million per year; 20,000 deaths).
Result from microorganisms in the hospital environment, weakened host status, and chain of transmission.
Compromised host: Individual with impaired resistance due to disease, therapy, or burns.

Control of Healthcare-Associated Infections

Handwashing
Disinfecting tubs used to bathe patients
Cleaning instruments scrupulously
Using disposable bandages and intubation
Infection control committees

Epidemiology

Study of Disease in Populations

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

Determines etiology of disease
Identifies factors concerning spread
Develops methods for control
Assembles data and graphs to outline incidence

Types of Epidemiological Investigation

Type	Description	Example
Descriptive	Collection and analysis of data regarding occurrence of disease	John Snow (cholera in London)
Analytical	Comparison of diseased and healthy groups to determine probable cause	Florence Nightingale (sanitation and typhus)
Experimental	Study using hypothesis and controlled experiments	Ignaz Semmelweis (handwashing and puerperal fever)

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
Morbidity rate:
Mortality rate: