Principles of Disease and Epidemiology: Study Notes

Principles of Disease and Epidemiology

Pathology, Etiology, and Pathogenesis

Understanding disease in microbiology requires clear definitions of key terms and concepts:

• Pathology: The scientific study of disease, including its causes, development, and effects on the body.

• Etiology: The study of the cause of a disease.

• Pathogenesis: The process by which disease develops in the body.

• Pathogens: Microorganisms that cause disease.

• Infection: The colonization of the body by pathogens.

• Disease: An abnormal state in which the body is not functioning normally.

Normal Microbiota and the Host

Normal and Transient Microbiota

The human body is home to a vast array of microorganisms, collectively known as the normal microbiota. These microbes are usually harmless and may even benefit the host. Transient microbiota are present temporarily and do not permanently colonize the host.

• Normal microbiota are found in multiple locations, such as the skin and large intestine.

• They form a symbiotic relationship with the host, where at least one party benefits.

Locations of Normal Microbiota

• Nose and throat (upper respiratory system)

• Eyes (conjunctiva)

• Mouth

• Skin

• Large intestine

• Urinary and reproductive systems (lower urethra in both sexes, vagina in females)

Microbial Antagonism and Probiotics

Microbial antagonism is the competition between microbes, where normal microbiota protect the host by:

• Occupying niches that pathogens might occupy

• Producing acids

• Producing bacteriocins (antibacterial proteins)

Probiotics are live microbes administered to confer a health benefit to the host.

Types of Symbiosis

• Amensalism: One species is harmed, and the benefit to the other is unclear (e.g., Staphylococcus aureus and Penicillium mold).

• Commensalism: One organism benefits, the other is unaffected (e.g., Streptococcus viridans in the mouth).

• Mutualism: Both organisms benefit (e.g., gut bacteria breaking down cellulose for rabbits).

• Parasitism: The parasite benefits at the host's expense (e.g., measles virus).

The Etiology of Infectious Diseases

Koch’s Postulates

Koch’s postulates are a series of steps used to determine the causative agent of a disease:

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

4. The pathogen must be re-isolated from the experimentally infected host and shown to be the original organism.

Exceptions to Koch’s postulates include pathogens that cannot be cultured in the lab or cause multiple diseases, and diseases that only affect humans.

Classifying Infectious Diseases

Symptoms, Signs, and Syndromes

• Symptom: A subjective change in body function felt by the patient (e.g., pain, fatigue).

• Sign: An objective, measurable change (e.g., fever, rash).

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

Types of Diseases

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

• Contagious disease: Easily spread from one host to another (e.g., measles).

• Noncommunicable disease: Not spread from host to host (e.g., tetanus).

Occurrence of Disease

• Incidence: Number of new cases in a specific time period.

• Prevalence: Total number of cases at a given time.

• Sporadic disease: Occurs occasionally.

• Endemic disease: Constantly present in a population.

• Epidemic disease: Many cases in a short time in a specific area.

• Pandemic disease: Worldwide epidemic.

• Herd immunity: Immunity in most of a population, often due to vaccination.

Severity or Duration of Disease

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

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

• Subacute disease: Intermediate between acute and chronic.

• Latent disease: Inactive for a period, then reactivates (e.g., shingles).

Spread of Infection

Reservoirs of Infection

A reservoir is a continual source of infection. Reservoirs can be:

• Human: Carriers may have inapparent or latent infections (e.g., AIDS, gonorrhea).

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

• Nonliving: Soil and water (e.g., botulism, tetanus).

Transmission of Disease

• Contact transmission:

  • Direct: Physical contact between hosts (e.g., kissing, touching).

  • Indirect: Via fomites (inanimate objects).

  • Droplet: Transmission via airborne droplets (e.g., coughing, sneezing).

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

• Vector transmission: Arthropods (e.g., mosquitoes, ticks) transmit pathogens by:

  • Mechanical transmission: Pathogen carried on vector's body.

  • Biological transmission: Pathogen reproduces in vector.

Epidemiology

Definition and Scope

Epidemiology is the study of where and when diseases occur and how they are transmitted in populations. The Centers for Disease Control and Prevention (CDC) is a key organization in the United States for epidemiological data and guidance.

• Case reporting: Health care workers report specified diseases to authorities.

• Nationally notifiable diseases: Diseases that must be reported by law.

• Morbidity: Number of people infected in a given time period.

• Mortality: Number of deaths in a given population in a given time period.

• Morbidity rate: $\text{Morbidity rate} = \frac{\text{Number of cases}}{\text{Total population}} \times 100$

• Mortality rate: $\text{Mortality rate} = \frac{\text{Number of deaths}}{\text{Total population}} \times 100$

Nosocomial (Hospital-Acquired) Infections

Definition and Risk Factors

Nosocomial infections are acquired during a hospital stay. Patients on antimicrobial therapy or immunosuppressants are at higher risk. These infections account for 5–15% of all hospital patients and are a leading cause of death in the US.

• Most commonly caused by Clostridium difficile.

• Result from the interaction of three factors:

  1. Presence of microorganisms in the hospital environment

  2. A compromised host

  3. A chain of transmission

Principal Sites of Nosocomial Infections

Type of Infection	Comments
Urinary tract infections	Most common; often related to catheter use.
Surgical site infections	Occur at or near surgical incisions.
Lower respiratory infections	Often associated with ventilators.
Bacteremia	May result from intravenous catheter use.
Other	Includes gastrointestinal and skin infections.

Emerging Infectious Diseases

Definition and Contributing Factors

Emerging infectious diseases are those that are new, increasing in incidence, or have the potential to increase. Contributing factors include:

• Genetic recombination (e.g., E. coli O157, avian influenza H5N1)

• Evolution of new strains (e.g., Vibrio cholerae O139)

• Inappropriate use of antibiotics and pesticides (leading to resistance)

• Changes in weather patterns (e.g., Hantavirus)

• Modern transportation (e.g., West Nile virus)

• Ecological disaster, war, expanding human settlement (e.g., coccidioidomycosis)

• Animal control measures (e.g., Lyme disease)

• Public health failure (e.g., diphtheria)