Epidemiology Flashcards
(20 cards)
John Snow – London Cholera Epidemic (1854)
- Contribution: Considered the father of modern epidemiology
- Study: Broad Street cholera outbreak in London, 1854
Key Points:
- Mapped cholera deaths to identify the source
- Compared households with different water sources
- Found cluster of cases around a contaminated water pump on Broad Street
- Hypothesized cholera was waterborne, not airborne
- Had the pump handle removed → outbreak subsided
- Used what we now call “epidemiological mapping”
- Also considered an early form of a double-blind observational study
Outcome:
- Supported the germ theory of disease
- Led to major changes in public health and sanitation
- Florence Nightingale worked nearby during the outbreak
Florence Nightingale (1858) – Pioneer of Epidemiology and Nursing
- Context: Crimean War (1850s)
- Contribution: Investigated causes of soldier deaths in military hospitals
Key Findings:
- Most deaths were not from battle wounds, but from infectious diseases
- Poor sanitation and hygiene were major contributors
- Used statistical analysis and visual data to support findings
Innovations:
- Created one of the first epidemiological studies
- Used a pie chart (coxcomb diagram) to display mortality data:
- Blue = preventable disease (e.g. typhus, cholera)
- Red = wounds
- Black = other causes
Impact:
- Prompted major reforms in military medical care
- Improved hospital hygiene and public health
- Helped establish nursing as a profession
- Conducted all this before germ theory was widely accepted
Define infection, contamination, disease, infectious disease
Infection
- Definition: Growth of a microorganism after successful colonization
- Key point: The organism is present and multiplying
- Example: Bacteria actively replicating on tissue → infection
Contamination
- Definition: Presence of a microorganism without growth or replication
- Key point: Organism is present but not multiplying
- Example: Staphylococcus on skin without causing harm → contamination
Disease
- Definition: Condition where normal body function or structure is disrupted
- May be caused by infection, toxins, or other processes
Infectious Disease
- Definition: Disease specifically caused by a microorganism
- Example: Tuberculosis, caused by Mycobacterium tuberculosis
Etiology and Koch’s Postulates
Etiology
- Definition: Study of the cause of disease
- The causative agent (or etiological agent) is the specific microorganism responsible for the disease
Koch’s Postulates
Used to determine whether a specific organism causes a specific disease
- The suspected pathogen must be present in all diseased individuals, and absent in healthy individuals
- The pathogen must be isolated and grown in pure culture
- The cultured agent must cause the same disease when introduced into a healthy, susceptible host
- The same agent must be re-isolated from the newly infected host
Modern Updates
- Some pathogens cannot be cultured, but can be identified using molecular methods (e.g. PCR to detect nucleic acid)
- Some healthy individuals may carry the organism asymptomatically, so Koch’s postulates are not absolute but still foundational
Incidence vs. Prevalence
Incidence
- Definition: Number (or proportion) of new disease cases in a population over a defined time period (e.g. per month, per year)
- Purpose:
- Tracks emergence and spread of disease
- Helps assess risk of infection
- Useful for detecting epidemics or pandemics
- Reporting:
- Can be shown as a number, percentage, or rate per 100,000 people
Prevalence
- Definition: Total number (or proportion) of all existing disease cases (both new and ongoing) in a population during a specific time period
- Purpose:
- Describes overall disease burden
- Helps understand chronic vs acute disease impact
- Reporting:
- May also be reported as a number, percentage, or standardized rate
Morbidity Rate vs. Mortality Rate
Morbidity Rate
- Definition: Proportion of a population that has a disease within a specified time period
- Formula:
→ Number of total disease cases / total population
- Purpose:
- Measures disease burden (how widespread a disease is)
- Can reflect chronic illness, not just acute cases
- Reporting:
- Often standardized to per 100,000 people for comparison between areas or populations
- Similar in concept to prevalence
Mortality Rate
- Definition: Proportion of a population that dies from a disease within a specified time period
- Formula:
→ Number of disease-related deaths / total population
- Purpose:
- Assesses severity or lethality of a disease
- Reporting:
- Also standardized to per 100,000 people
Case Fatality Rate (CFR)
- Definition: Proportion of infected individuals who die from a specific disease during a specified time period
-
Formula:
CFR = (Number of deaths due to the disease) / (Total number of cases of the disease) -
Purpose:
- Measures severity or lethality of a disease
- Often used during outbreaks or epidemics to assess risk
-
Note:
- CFR applies only to the infected population, not the general population
- Can vary depending on factors like healthcare access, virulence, and host factors
-
Reporting:
- Sometimes expressed as a percentage or standardized per 100,000
Patterns of incidence
Disease Outbreak
Outbreak (General Definition)
- A sudden increase in the number of cases of a disease
- Refers to a transmission or acquisition event, not the type of disease
Point Source Outbreak
- Definition: All infected individuals are exposed to the same source
- Transmission: Typically not person-to-person (e.g., contaminated food or water)
- Features:
- Localized and short-term
- Easier to control
- Example: Foodborne illness from a contaminated food court meal over Memorial Day weekend
Propagated Outbreak
- Definition: Disease is spread from person to person (communicable)
- Transmission: Each infected person becomes a new source
- Features:
- Harder to control, often longer-lasting
- Requires isolation/quarantine to limit spread
- Example: COVID-19, chickenpox at a party
Note: Not all outbreaks become epidemics or pandemics, but all represent an abnormal rise in case number.
Reservoirs of Infection
- Definition: Natural habitat where a pathogen is maintained over time
- Purpose: Acts as a source from which other individuals can become infected
-
Types:
-
Non-living reservoirs:
- Soil (e.g., Clostridium tetani)
- Water (e.g., Vibrio cholerae)
-
Living reservoirs:
- Animals
- Humans
-
Non-living reservoirs:
Animal Reservoirs
- Term: Zoonoses (zoonotic diseases)
- Definition: Diseases transmitted from animals to humans
- Examples: Rabies, plague, salmonellosis
- Animals act as primary reservoir even if not always symptomatic
Human Reservoirs
- Term: Carrier
- Definition: A human who harbors and transmits a pathogen
- Note: “Carrier” is only used for human reservoirs
- Types:
1. Active carrier – symptomatic and infectious
2. Asymptomatic carrier – no symptoms, but still infectious (e.g., Typhoid Mary)
3. Passive carrier – temporarily carries pathogen without infection (e.g., contaminated hands or clothing)
Key Point
- Difference between reservoir and source:
- A reservoir sustains the pathogen long-term in nature
- A source is the immediate origin from which a host is infected (can be the reservoir, but not always)
Carriers of Disease
- Definition: Individuals who harbor and transmit pathogens, may or may not show symptoms
- Applies only to human reservoirs
Active Carrier
- Definition: A person who knows they are infected
- Features:
- Actively infected
- Shows signs and symptoms
- Capable of transmitting disease
Asymptomatic Carrier
- Definition: A person who is infected but does not know
- Features:
- No visible symptoms
- Can still transmit disease
- Often undiagnosed → risk of unnoticed spread
Passive Carrier
- Definition: A person who is contaminated but not infected
- Features:
- Does not harbor the pathogen internally
- Serves as a mechanical vector (e.g., hands, clothing)
- Example: Healthcare workers not washing hands or changing gloves between patients
Clinical Note
- Key distinction:
- Active/Asymptomatic = infected
- Passive = not infected, only contaminated
Transmissibility of Disease
- Key Concept: Not all infectious diseases are transmissible between people
Non-communicable Disease
- Definition: Infectious disease that is not transmitted person-to-person
- Transmission: Acquired from environmental reservoirs, not people
- Examples:
- Botulism (via toxins in food)
- Tetanus (via spores in soil)
- Bubonic plague (via fleas from rodents)
Communicable Disease
- Definition: Infectious disease that can be transmitted between people
- Transmission: Via direct or indirect contact (respiratory droplets, bodily fluids, fomites, etc.)
- Examples:
- Syphilis, common cold, influenza, pneumonic plague
- Note: This is a broad category that includes contagious diseases
Contagious Disease
Highly transmissible communicable disease -> Spreads easily, often with no direct contact required (e.g. airborne)
- Examples:
- Measles, chickenpox
- Note: All contagious diseases are communicable, but not all communicable diseases are contagious
Important Distinctions
- Non-communicable: No person-to-person spread
- Communicable: Can spread between people
- Contagious: Spreads easily and rapidly between people
- Example contrast:
- HIV = communicable but not contagious (requires specific contact)
- Measles = both communicable and contagious
Clinical Note
- Terms are often used loosely in public discourse, but in microbiology, the distinctions matter
Types of Contact transmission
- Direct
- Indirect
- Droplet Transmission
- Airborne Transmission
Direct Contact Transmission
- Definition: Person-to-person spread via physical contact
-
Subtypes:
-
Vertical: Parent → child (pregnancy, birth, breastfeeding)
- e.g., Congenital syphilis
-
Horizontal: Between any two people, not during childbirth
- e.g., Syphilis (non-congenital)
-
Vertical: Parent → child (pregnancy, birth, breastfeeding)
Indirect Contact Transmission
- Definition: Disease transmitted via inanimate intermediary
- Intermediary: Called fomite (e.g., surfaces, objects)
- Transmission route: Person → fomite → person
-
Examples:
- Public keyboards, vending buttons, shared needles
- Note: Some viruses (e.g., norovirus) can survive months on surfaces
Droplet Transmission
- Definition: Pathogen carried in respiratory droplets
- Source: Coughing or sneezing
- Range: Transmitted within 1 meter
- Note: Large droplets fall quickly; close proximity required
Airborne Transmission
- Definition: Pathogen carried in respiratory droplets
- Source: Coughing or sneezing
- Range: Transmitted beyond 1 meter
- Note: Small particles remain suspended and travel farther
Types of Vehicle transmission
Vehicle Transmission (General)
- Definition: Transmission of disease via a shared medium like food, water, or air
- Mechanism: Pathogen is ingested or inhaled, not passed via direct contact
Foodborne Transmission
- Definition: Pathogen is ingested via contaminated food
- Examples:
- Salmonella, E. coli, Listeria
- Sources: Undercooked meat, unwashed produce, cross-contamination
Waterborne Transmission
- Definition: Pathogen is ingested via contaminated water
- Examples:
- Vibrio cholerae, Giardia, Cryptosporidium
- Sources: Inadequate water treatment, sewage contamination
Aerosol Transmission
- Definition: Tiny particles (not droplets) containing pathogens are suspended in air and inhaled
- Key Feature: Not the same as airborne transmission (no respiratory droplets involved)
- Examples:
- Fungal spores, environmental bacteria (e.g., Legionella)
- Note: Often contagious due to long suspension time in atmosphere
Vector Transmission (General)
- Definition: Transmission of disease via insects or arthropods
- Types:
1. Mechanical vector
2. Biological vector
Mechanical Vector Transmission
- Definition: Pathogen is carried on the surface (e.g., exoskeleton) of the insect
- Transmission: No biting; passive transfer to food or surfaces
- Examples:
- Flies, cockroaches
- Fly lands on feces → lands on food → person eats food
- Note: Can lead to foodborne transmission (linked pathway)
Biological Vector Transmission
- Definition: Pathogen is found inside the insect and is transmitted by bite
- Process:
1. Pathogen enters vector during feeding
2. Multiplies inside the vector
3. Passed to new host via saliva during next bite
- Examples:
- Mosquitoes (e.g., malaria, dengue)
- Ticks (e.g., Lyme disease)
- Fleas (e.g., plague)
Key Distinction
- Mechanical = outside of insect, no biological involvement
- Biological = inside insect, pathogen develops/reproduces in vector
Common Vectors and disease
