BIOL203.Chapter14
Principles of Disease and Epidemiology
1. Definitions
Pathology: The study of disease.
Etiology: The study of the cause of a disease.
Pathogenesis: The development of disease.
Infection: Colonization of the body by pathogens.
Disease: An abnormal state where the body does not function normally.
2. Microbiota
2.1 Normal and Transient Microbiota
Normal Microbiota: Permanently colonize the host.
Transient Microbiota: Present for days, weeks, or months.
2.2 Symbiosis
Commensalism: One organism benefits; the other is unaffected.
Example: S. epidermidis on skin.
Mutualism: Both organisms benefit.
Example: E. coli in the large intestine (produces vitamin K).
Parasitism: One benefits at the expense of the other.
Example: Disease-causing bacteria and viruses.
Some normal microbiota can be opportunistic pathogens (e.g., E. coli).
2.3 Microbial Antagonism
Competition between microbes helps protect the host.
Mechanisms include:
Occupying niches that pathogens may occupy.
Producing acids and bacteriocins.
3. Koch's Postulates
Postulate 1: The same pathogen must be present in every case of the disease.
Postulate 2: The pathogen must be isolated from the diseased host and grown in pure culture.
Postulate 3: The cultured pathogen must cause the disease when inoculated into a healthy host.
Postulate 4: The pathogen must be isolated from the inoculated host and shown to be the original organism.
4. Classifying Infectious Diseases
4.1 Types of Diseases
Communicable Disease: Spread from one host to another.
Contagious Disease: Easily spread communicable disease.
Noncommunicable Disease: Not transmitted between hosts.
4.2 Disease Occurrence
Incidence: Fraction of a population that contracts a disease at a specific time.
Prevalence: Fraction having a specific disease at a given time.
Sporadic Disease: Occasional cases in a population.
Endemic Disease: Constantly present in a population (e.g., common cold).
Epidemic Disease: Many hosts in a given area in a short time (e.g., flu).
Pandemic Disease: Worldwide epidemic (e.g., AIDS).
Herd Immunity: Immunity in most of the population.
5. Severity and Duration of Diseases
5.1 Types of Disease Duration
Acute Disease: Rapid onset of symptoms (e.g., flu).
Chronic Disease: Develops slowly (e.g., tuberculosis).
Subacute Disease: Intermediate severity.
Latent Disease: Period of no symptoms (e.g., shingles).
5.2 Extent of Host Involvement
Local Infection: Limited to a small area (e.g., abscess).
Systemic Infection: Infection throughout the body (e.g., measles).
Sepsis: Toxic inflammatory condition from microbes spreading.
6. Transmission of Disease
6.1 Reservoirs of Infection
Human Reservoirs: Inapparent infections (e.g., AIDS).
Animal Reservoirs: Zoonoses (e.g., rabies, Lyme disease).
Nonliving Reservoirs: Soil, water (e.g., tetanus).
6.2 Transmission Methods
Contact Transmission: Direct (person to person) or indirect (via fomites).
Vehicle Transmission: Through inanimate reservoirs (e.g., food, water).
Vector Transmission: Arthropods involved in disease spread (mechanical or biological).
7. Nosocomial Infections
Acquired during hospital stay, affecting 5–15% of patients.
Common types include urinary tract infections and surgical site infections, often related to procedures.
8. Emerging Infectious Diseases
Diseases that are new or increasing in incidence.
8.1 Contributing Factors
Genetic recombination, inappropriate antibiotic use, climate change, modernization, etc.
9. Epidemiology
9.1 CDC's Role
Collects and analyzes disease information and publishes MMWR (Morbidity and Mortality Weekly Report).
9.2 Types of Epidemiology
Descriptive: Analyzes data on disease occurrence.
Analytical: Compares diseased versus healthy groups.
Experimental: Controlled hypothesis-testing experiments.
10. Important Epidemiological Concepts
Morbidity Rate: Affected individuals in relation to total population.
Mortality Rate: Deaths from a disease in relation to the population.