Principles of Disease and Epidemiology: Microbiology Study Notes

Principles of Disease and Epidemiology

Pathology, Infection, and Disease

Pathology is the scientific study of disease, focusing on the cause (etiology), development (pathogenesis), and the structural and functional changes in the body. Infection refers to the invasion or colonization of the body by pathogenic microorganisms, while disease is the abnormal state resulting from infection, where normal body functions are disrupted.

Normal Microbiota

Establishment and Distribution

Humans are generally free of microbes in utero, but at birth, normal microbiota begin to establish. The first exposure is often to Lactobacillus from the mother's vagina, which colonizes the newborn's intestines. Microbiota are then acquired from the environment and remain throughout life. The human body contains about 10 times more bacterial cells than human cells.

• Normal microbiota (normal flora): Permanent residents that do not cause disease under normal conditions.

• Transient microbiota: Present temporarily and then disappear.

Distribution and composition are influenced by nutrients, physical and chemical factors (temperature, pH, O2, CO2, salinity, sunlight), host defenses, and mechanical factors (chewing, GI flow, urine flushing). Other factors include age, diet, health, hygiene, and lifestyle.

Relationships Between Normal Microbiota and the Host

• Microbial antagonism (competitive exclusion): Normal microbiota prevent overgrowth of harmful organisms by competing for nutrients, producing inhibitory substances, and altering environmental conditions.

• Symbiosis: Relationship between two organisms where at least one benefits.

  • Commensalism: One benefits, the other is unaffected (e.g., Staphylococcus epidermidis).

  • Mutualism: Both benefit (e.g., Escherichia coli in the intestine synthesizes vitamins for the host).

  • Parasitism: One benefits at the expense of the other (many pathogens).

• Opportunistic microorganisms: Normally harmless but can cause disease in altered environments or immunocompromised hosts.

Etiology of Infectious Diseases

Koch’s Postulates

Robert Koch established criteria to identify the causative agent of infectious diseases:

1. The same pathogen must be present in every case of the disease.

2. The pathogen must be isolated and grown in pure culture.

3. The cultured pathogen must cause disease in a healthy host.

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

Exceptions include pathogens with unique culture requirements, diseases caused by multiple organisms, pathogens causing multiple diseases, and ethical limitations (e.g., HIV).

Classifying Infectious Diseases

Symptoms, Signs, and Syndromes

• Symptoms: Subjective changes (e.g., pain, malaise).

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

• Syndrome: Specific group of signs and symptoms associated with a disease.

Communicability

• Communicable disease: Spreads from host to host (e.g., chickenpox, tuberculosis).

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

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

Occurrence of Disease

• Incidence: Number of new cases in a time period (indicator of spread).

• Prevalence: Total number of cases at a given time (indicator of how widespread and long-lasting).

Classification by Frequency

• Sporadic: Occurs occasionally (e.g., typhoid fever).

• Endemic: Constantly present (e.g., common cold).

• Epidemic: Many cases in a short period (e.g., influenza).

• Pandemic: Worldwide epidemic (e.g., AIDS, COVID-19).

Severity or Duration

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

• Chronic: Slow development, long duration (e.g., tuberculosis).

• Subacute: Intermediate between acute and chronic.

• Latent: Inactive for a time, then active (e.g., shingles).

Population immunity (herd immunity) can limit disease spread, especially through vaccination.

Extent of Host Involvement

• Local infection: Limited to a small area (e.g., boils).

• Systemic infection: Spread throughout the body (e.g., measles).

• Focal infection: Local infection spreads to other areas.

• Sepsis: Toxic inflammatory condition from spread of microbes or toxins.

• Septicemia: Pathogens multiplying in blood (blood poisoning).

• Bacteremia: Bacteria in blood.

• Toxemia: Toxins in blood.

• Viremia: Viruses in blood.

• Primary infection: Initial acute infection.

• Secondary infection: Caused by opportunists after primary infection weakens defenses.

• Subclinical infection: No noticeable illness.

Patterns and Development of Disease

Sequence of Disease Development

Infectious diseases follow a sequence: reservoir, transmission, invasion, pathogenesis, and host resistance. Predisposing factors (e.g., age, nutrition, genetics, environment) can increase susceptibility.

Stages of Disease

• Incubation period: Time between infection and first symptoms; depends on organism, virulence, dose, and host resistance.

• Prodromal period: Short period with mild, general symptoms.

• Period of illness: Most severe signs and symptoms; immune response is critical.

• Period of decline: Signs and symptoms subside; risk of secondary infection.

• Period of convalescence: Recovery and return to normal; pathogen may still be spread.

Spread of Infection

Reservoirs of Infection

• Human reservoirs: People with disease or carriers (e.g., AIDS, hepatitis).

• Animal reservoirs: Zoonoses are diseases transmitted from animals to humans (e.g., rabies, Lyme disease).

• Nonliving reservoirs: Soil (e.g., Clostridium), water (e.g., Vibrio cholerae), and food.

Transmission of Disease

• Contact transmission: Direct (person-to-person), indirect (via fomites), or droplet (short-range airborne).

• Vehicle transmission: Via water, food, air, blood, or other fluids.

• Vector transmission: Arthropods (e.g., insects) transmit pathogens mechanically (on body) or biologically (bite and pathogen development in vector).

Nosocomial (Hospital-Acquired) Infections

Overview and Risk Factors

Nosocomial infections (now called healthcare-associated infections, HAIs) are acquired in healthcare settings. They result from the interaction of microorganisms in the hospital, compromised hosts, and transmission chains.

Microorganisms Involved

Microorganism	Percentage of Total Infections	Percentage Resistant to Antibiotics	Infections Caused
Coagulase-negative staphylococci	15%	89%	Sepsis
Staphylococcus aureus	15%	60%	Surgical wound infections
Enterococcus	10%	29%	Surgical wound and urinary tract infections
Escherichia coli, Pseudomonas aeruginosa, Enterobacter, Klebsiella	23%	3-32%	Surgical wound and urinary tract infections
Clostridium difficile	13%	Not reported	Diarrhea
Fungi (mainly Candida albicans)	6%	Not reported	Urinary tract infections and sepsis
Other gram-negative bacteria (Acinetobacter, Citrobacter, Haemophilus)	7%	Not reported	Urinary tract and wound infections

Most Common Sites of Nosocomial Infections

Site	Percentage
Urinary tract infections	40%
Surgical site infections	20%
Lower respiratory infections	15%
Bacteremia (IV catheterization)	11%
Cutaneous infections	8%
Other	6%

Control of Nosocomial Infections

• Use aseptic techniques and handle contaminated materials carefully.

• Frequent and thorough handwashing is the most important preventive measure.

• Educate staff, use isolation rooms, and reduce patient exposure to pathogens.

Emerging Infectious Diseases (EIDs)

EIDs are new or changing diseases with increasing incidence or potential to increase. About 75% are zoonotic, mainly viral. Examples include Ebola, MERS-CoV, and drug-resistant tuberculosis.

Epidemiology

Epidemiology is the science of when, where, and how diseases occur and are transmitted in populations.

• Descriptive epidemiology: Collects data on disease occurrence.

• Analytical epidemiology: Determines probable cause of disease.

• Experimental epidemiology: Tests hypotheses about disease causes through controlled experiments.