2: Why do diseases happen? Flashcards
What are the 5 methods used to understand disease?
-Koch’s postulates
-Hill’s criteria
-Evan’s theory
-Epidemiological triad
-Causal diagrams
What is etiology?
The study or theory of the factors that cause disease
What is the difference between primary and secondary disease determinants (causative factors)?
Primary: Major effect in inducing disease
Secondary: Predisposing, enabling and reinforcing factors
What are some primary disease determinants?
- Exposure to the virus
- Genetic factors
What are some secondary disease determinants?
- Gender
- Age
- Lifestyle
Primary and secondary determinants can be _________ or ___________
Intrinsic; extrinsic
What are some examples of intrinsic determinants?
- Genetics
- Age
- Metabolism
- Hormonal status
- Immunological status
What are some examples of extrinsic determinants?
- Infectious agents (viruses, bacteria, parasites, fungi)
- Non-infectious agents (toxins, poisons, heat/cold injuries)
- Location
- Climate
What is a monofactorial disease determinant?
A single factor related to disease
What is a multifactorial disease determinant?
Interaction between factors
What did some of Koch’s work include?
- Identified anthrax bacillus from sheep and injected healthy sheep
- Developed observation methods, pure culture and staining methods
- Identified Mycobacterium tuberculosis as cause of tuberculosis
What is Koch’s 1st postulate?
- The specific organism should be shown to be present in all cases of animals suffering from a specific disease, but should not be found in healthy animals
What is Koch’s 2nd postulate?
- The specific microorganism should be isolated from the diseased animal and grown in pure culture on artificial laboratory media
What is Koch’s 3rd postulate?
- The freshly isolated microorganism, when inoculated into a healthy laboratory animal, should cause the same disease seen in the original animal
What is Koch’s 4th postulate?
- The microorganism should be re-isolated in pure culture from the experimental animal
Why must culture results be interpreted carefully?
Most microorganisms don’t cause disease even though they are present
What are Hill’s criteria for plausible causality? (sorry there’s a lot)
Temporality
Strength
Biological gradient
Consistency
Specificity
Plausibility
Coherence
Analogy
Experiment (setting)
What is temporality?
The cause must precede the outcome
What is strength?
Strong relationship between variables (large effect size)
What is a biological gradient?
Dose-response: more exposure leads to more outcome
What is consistency?
The relationship is consistent in different studies and populations (involves stats)
What is specificity?
Single cause for a single effect
What is plausibility?
There is a biological rationale for the relationship
What is coherence?
Relationship is consistent with previous knowledge
What is analogy?
The relationship is comparable to other similar relationships
What is the experiment (setting) criteria?
Strong criteria - randomly assigned treatment changes the clinical outcome (change in cause = change in effect)
What are the 8 Evan’s criteria? (sorry again)
- The incidence and/or prevalence should be higher in exposed individuals than non-exposed
- The exposure should be more common in cases with than in those without the disease
- Exposure must precede the disease
- There should be a spectrum of measurable host response to the agent
- Elimination of the cause should lower the incidence of the disease
- Modifying the hosts response should decrease expression of the disease
- Disease should be reproducible experimentally
- All findings should make biological and epidemiological sense
What is a causal model?
A is the primary determinant and B, C, D are secondary determinants. A+B+C+D together is enough to cause disease
What makes up the epidemiological triad?
Agent
Host
Environment
What is pathogenicity?
Ability of an infectious agent to cause disease in a particular hose under specific circumstances
What is virulence?
Degree to which an infectious agent causes clinical disease in a particular host; expresses the severity
What is a disadvantage of a causal pie?
It does not capture the dynamics of cause and effect very well
What is an advantage of the epidemiological triad?
It focuses on the relationships between host, pathogen and environment
What is the fundamental assumption of epidemiology?
Diseases do not distribute randomly in populations, but rather distribute in relation to their determinants
