Principles of Disease and Epidemiology – Microbiology Study Notes

Principles of Disease and Epidemiology

Pathology, Infection, and Disease

This section introduces the foundational concepts of disease in microbiology, focusing on the study of disease, its causes, and how infections develop.

• 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 results in a change from a state of health.

Note: An infection may exist without causing detectable disease, and disease can result when microorganisms are found in parts of the body where they are not normally present.

The Human Microbiome

The human microbiome consists of all the microorganisms that reside on and within the human body, playing a crucial role in health and disease.

• Acquisition: Begins in utero (placental microbiome), continues during birth (especially vaginal birth), and expands through contact with food, people, and pets.

• Normal microbiota (normal flora): Microorganisms that permanently colonize the host without causing disease under normal conditions.

• Transient microbiota: Microorganisms that are present temporarily and then disappear.

• Factors influencing microbiota: Nutrients, physical/chemical factors, mechanical factors, host defenses, age, diet, geography, hygiene, and lifestyle.

• Role in immunity: Normal microbiota contribute to the development and function of the immune system.

Representative Normal Microbiota by Body Region

Different body regions host distinct microbial communities, each adapted to local conditions.

Region	Principal Components	Comments
Skin	Propionibacterium, Staphylococcus, Corynebacterium, Micrococcus, Acinetobacter, Brevibacterium, Candida (fungus), Malassezia (fungus)	Antimicrobial secretions, low pH, low moisture
Eyes (Conjunctiva)	Staphylococcus epidermidis, S. aureus, diphtheroids, Propionibacterium, Corynebacterium, streptococci, Micrococcus	Tears and blinking remove microbes; lysozyme present
Nose and Throat	S. aureus, S. epidermidis, diphtheroids, Streptococcus pneumoniae, Haemophilus, Neisseria	Microbial antagonism, mucus, ciliary action
Mouth	Streptococcus, Lactobacillus, Actinomyces, Bacteroides, Veillonella, Neisseria, Haemophilus, Fusobacterium, Treponema, Staphylococcus, Corynebacterium, Candida (fungus)	Moist, warm, food-rich environment; saliva contains antimicrobials
Large Intestine	E. coli, Bacteroides, Fusobacterium, Lactobacillus, Enterococcus, Bifidobacterium, Enterobacter, Citrobacter, Proteus, Klebsiella, Candida (fungus)	Largest numbers of microbiota; prevents pathogen growth
Urinary/Genital Systems	Staphylococcus, Micrococcus, Enterococcus, Lactobacillus, Bacteroides, diphtheroids, Pseudomonas, Klebsiella, Proteus, Candida albicans (fungus), Trichomonas vaginalis (protozoan)	Acidic secretions, urine flow, mucus, cilia

Microbial Interactions and Symbiosis

Microorganisms interact with their host in various ways, ranging from harmless coexistence to disease-causing relationships.

• Microbial antagonism (competitive exclusion): Normal microbiota compete with pathogens for nutrients, produce harmful substances, and alter local pH and oxygen levels.

• Symbiosis: A close relationship between two organisms, classified as:

  • 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 (e.g., pathogenic bacteria).

• Opportunistic pathogens: Normally harmless microbes that can cause disease under certain conditions (e.g., weakened immunity).

Koch’s Postulates and Etiology of Infectious Diseases

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 cultured pathogen must cause the disease when introduced into a healthy, susceptible host.

• The pathogen must be re-isolated from the experimentally infected host 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

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

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

• Signs: Objective, measurable changes (e.g., fever, rash).

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

• Communicable disease: Spread from one host to another (e.g., influenza).

• Contagious disease: Easily and rapidly spread (e.g., measles).

• Noncommunicable disease: Not spread between hosts (e.g., tetanus).

Occurrence and Severity of Disease

Understanding disease occurrence and severity is essential for epidemiology and public health.

• 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.

• Endemic disease: Constantly present in a population.

• Epidemic disease: Many people acquire the disease in a short time.

• Pandemic disease: Worldwide epidemic.

• Acute disease: Rapid onset, short duration (e.g., influenza).

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

• Subacute disease: Intermediate between acute and chronic.

• Latent disease: Pathogen remains inactive for a time, then activates (e.g., herpes).

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

Fatality Ratios

Fatality ratios help assess the lethality of infectious diseases.

• Infection Fatality Ratio (IFR): $\text{IFR} = \frac{\text{deaths from the disease}}{\text{total infections}}$

• Case Fatality Ratio (CFR): $\text{CFR} = \frac{\text{deaths from the disease}}{\text{confirmed cases}}$

Aspect	CFR	IFR
Counts	Confirmed cases only	All infections
Includes asymptomatic cases	No	Yes
Ease of calculation	Easier	Harder
Typical size	Higher	Lower
Usefulness	Clinical severity	Population-level risk

Extent of Host Involvement

Infections can be classified by their distribution in the body.

• Local infection: 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 the spread of microbes or their 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: Acute infection causing the initial illness.

• Secondary infection: Opportunistic infection after a primary infection.

• Subclinical infection: No noticeable signs or symptoms.

Patterns of Disease

The development of disease follows a predictable sequence of stages.

• Incubation period: Time 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: Recovery; body returns to prediseased state.

Reservoirs and Transmission of Infection

Reservoirs are continual sources of infection, and transmission can occur through various routes.

• Human reservoirs: People with active disease or carriers (asymptomatic, incubating, convalescent, chronic, passive).

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

• Nonliving reservoirs: Soil, water, and food.

Transmission Methods

• Contact transmission: Direct (person-to-person), indirect (fomites), droplet (airborne droplets <1 meter).

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

• Vector transmission: Arthropods (fleas, ticks, mosquitoes) transmit disease mechanically (on body parts) or biologically (pathogen reproduces in vector).

Healthcare-Associated Infections (HAIs)

HAIs, or nosocomial infections, are acquired during treatment in healthcare settings and are a significant public health concern.

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

• Common pathogens: Staphylococcus aureus, Clostridioides difficile, Enterococcus spp., Escherichia coli, Candida auris, Pseudomonas aeruginosa, Klebsiella pneumoniae, Acinetobacter baumannii.

• Prevention: Universal and standard precautions (hand hygiene, PPE, disinfection), transmission-based precautions (contact, droplet, airborne), and infection control committees.

Emerging Infectious Diseases

Emerging infectious diseases are new, changing, or increasing in incidence, often zoonotic and vector-borne.

• Contributing factors: Genetic recombination, evolution, antibiotic/pesticide use, climate change, transportation, ecological changes, animal control, public health failures, bioterrorism.

• Examples: E. coli O157:H7, avian influenza, Zika virus, West Nile virus, antibiotic-resistant bacteria.

Epidemiology

Epidemiology is the study of disease occurrence, distribution, and transmission in populations.

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

• Analytical epidemiology: Analysis to determine cause or risk factors (e.g., Florence Nightingale's work).

• Experimental epidemiology: Hypothesis testing with controlled experiments (e.g., Semmelweis's handwashing study).

• Key terms: Morbidity (illness rate), Mortality (death rate), Notifiable diseases (must be reported to health authorities).

Additional info: The Centers for Disease Control and Prevention (CDC) is the primary U.S. agency for monitoring and controlling infectious diseases.