Principles of Disease and Epidemiology – Study Notes

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Principles of Disease and Epidemiology

Pathology, Infection, and Disease

Understanding the principles of disease and epidemiology is fundamental in microbiology. This section introduces key definitions and concepts related to disease processes.

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 leads to a change in health status. Notably, infection can exist without overt disease, and disease can occur if microbes colonize unusual body sites.

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.

Establishment: Begins in utero, with placental microbiome including Enterobacteriaceae and Propionibacterium. Lactobacilli colonize the neonate's intestine during vaginal birth.
Microorganisms are further acquired from food, people, and pets, and persist throughout life, fluctuating in number and composition.
Estimated size: Approximately bacterial cells.
Human Microbiome Project: Analyzes the relationships between microbial communities and human health.
Normal microbiota (normal flora): Permanently colonize the host without causing disease under normal conditions.
Transient microbiota: Present temporarily (days to months) before disappearing.

Factors Influencing the Normal Microbiota

Nutrients
Physical and chemical factors
Mechanical factors
Body's defenses
Age, diet, nutritional status, geography, personal hygiene, living conditions, occupation, and lifestyle
Normal microbiota contribute to immune system development.

Representative Normal Microbiota by Body Region

Region	Principal Components	Comments
Skin	Propionibacterium, Staphylococcus, Corynebacterium, Micrococcus, Acinetobacter, Brevibacterium, Malassezia (fungus), Candida (fungus)	Most microbes in direct contact with skin; keratin is a barrier; low pH inhibits many microbes.
Eyes (Conjunctiva)	Staphylococcus epidermidis, S. aureus, Diphtheroids, Propionibacterium, Corynebacterium, Streptococcus	Tears and blinking eliminate some microbes; continuation of skin or nasal cavity flora.
Nose and Throat (Upper Respiratory System)	S. aureus, S. epidermidis, Streptococcus pneumoniae, Haemophilus, Neisseria	Microbial population reduced by mucous and ciliary action.
Mouth	Streptococcus, Lactobacillus, Actinomyces, Bacteroides, Veillonella, Neisseria, Haemophilus, Fusobacterium, Treponema, Staphylococcus, Corynebacterium, Candida (fungus)	Moist, warm, and nutrient-rich; saliva contains antimicrobial substances.
Large Intestine	Bacteroides, Fusobacterium, Lactobacillus, Clostridium, Enterococcus, Escherichia, Citrobacter, Proteus, Klebsiella, Candida (fungus)	Largest microbial population; mucus and periodic shedding limit overgrowth.
Urinary and Genital Systems	Staphylococcus, Micrococcus, Enterococcus, Lactobacillus, Streptococcus, Pseudomonas, Klebsiella, Proteus, Enterobacter, Gardnerella, Mycobacterium, Trichomonas vaginalis (protozoan), Candida (fungus)	Urine flow and vaginal acidity inhibit microbial growth.

Birth and Microbiome

Vaginal birth: Lactobacillus and Bacteroides are prevalent in the neonate's microbiome.
Cesarean birth: Microbiome resembles human skin; Staphylococcus aureus is common.

Relationships Between the Normal Microbiota and the Host

Normal microbiota interact with the host in various ways, influencing health and disease susceptibility.

Microbial antagonism (competitive exclusion): Normal microbiota compete with pathogens for nutrients, produce inhibitory substances, and alter environmental conditions (e.g., pH, oxygen).
Clostridium difficile: Can cause severe intestinal infections if normal microbiota are disrupted (e.g., by antibiotics). Fecal microbial transfer (FMT) can restore healthy microbiota.

Types of Symbiosis

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 synthesizes vitamins for the host).
Parasitism: One organism benefits at the expense of the other; many pathogens are parasites.
Some normal microbiota are opportunistic pathogens, causing disease under certain conditions.

Koch's Postulates

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 pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible animal.
The pathogen must be re-isolated from the inoculated animal and shown to be the original organism.

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

Classifying Infectious Diseases

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 characterize a disease.
Communicable disease: Spread from one host to another (e.g., COVID-19, influenza).
Contagious disease: Easily and rapidly spread between hosts.
Noncommunicable disease: Not spread from host to host (e.g., tetanus).

Occurrence of a Disease

Incidence: Number of new cases in a specific time period.
Prevalence: Total number of cases (old and new) 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 a Disease

Duration: Average time from diagnosis to cure or death.
Acute disease: Rapid onset, short duration (e.g., influenza).
Chronic disease: Slow development, long duration (e.g., tuberculosis).
Subacute disease: Intermediate between acute and chronic.
Latent disease: Pathogen inactive for a time, then activates (e.g., herpes simplex virus).
Herd immunity: Immunity in most of a population, reducing disease spread.

Severity: Extent and seriousness of disease; can range from asymptomatic to critical.
Sepsis: Extreme inflammatory response to infection.
Infection fatality ratio (IFR):
Case fatality ratio (CFR):

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 (blood poisoning).
Toxemia: Toxins in the blood.
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.

Predisposing Factors

Factors that increase susceptibility to disease or alter its course include:

Nutrition
Sex
Genetic inheritance
Climate and environment
Vaccination status
Age
Lifestyle and behaviors
Compromised host status

Development of Disease

Incubation period: Time between initial infection and first symptoms.
Prodromal period: Short period with early, mild symptoms.
Period of illness: Disease is most severe.
Period of decline: Signs and symptoms subside.
Period of convalescence: Recovery; body returns to prediseased state.