Epidemiology Notes

Puerperal fever: A bacterial infection of the uterus.
Statistics: In the mid-19th century, 1 in 8 women died from this infection in Vienna hospitals following childbirth.
Ignaz Semmelweis' Observations:
- Noted that incidence in a section handled by medical students was 4 times higher than in a section run by midwives.
- A friend’s death from a scalpel wound echoed similar symptoms of puerperal fever.
- Hypothesized that a "poison" was transferred by medical students to women in childbirth.
Germ Theory Foundations:
- This insight occurred before Pasteur and Koch's germ theory was widely accepted; the pathogen was likely Streptococcus pyogenes.
- Implemented handwashing with disinfectants, leading to a drop in incidence by 1/3.

Definition: Epidemiology is the study of disease patterns in populations.
Epidemiologists: Health detectives who gather data on disease sources and risk factors.
Objectives:
- Design strategies for infection control.
- Predict and prevent disease spread.
Disciplines Involved:
- Ecology, microbiology, sociology, statistics, psychology.
Daily Habits Informed by Epidemiology:
- Handwashing and waste disposal practices.

Communicable Diseases:
- Transmitted from one host to another (e.g., measles, influenza).
- Transmission involves interactions between the environment, pathogen, and host.
- Control measures: Improve sanitation, vaccination, antimicrobial medications.
Non-communicable Diseases:
- Not spread between hosts; often arise from an individual's microbiota or environment (e.g., Clostridium tetani).

Rates of Disease:
- Focus on rates over absolute numbers; understand context (e.g., small vs large city).
- Attack Rate: Percentage becoming ill after exposure; reflects infectious dose and immunity.
- Incidence Rate: New cases/time/population; measures individual risk.
- Prevalence: Total cases at a specific time/period; indicates disease impact on society.
- Morbidity: Incidence of disease for at-risk populations (e.g., high morbidity for influenza).
- Mortality: Overall death rate; often linked with non-communicable diseases in developed countries.
- Case-Fatality Rate: Percentage of deaths from a specific disease.

Endemic: Constant presence of a disease (e.g., common cold).
Epidemic: Unusually high cases, may originate from introduced or endemic sources.
Outbreak: Cluster of cases in a specific timeframe.
Pandemic: Global spread of a disease (e.g., AIDS).

Definition: Natural habitats where pathogens live (in humans, animals, or the environment).
Human Reservoirs:
- Can be symptomatic/asymptomatic; easier to control (e.g., smallpox).
- Some individuals can transmit diseases without symptoms (e.g., Neisseria gonorrhoeae).
Animal Reservoirs:
- Diseases (e.g., rabies) primarily exist in animals but can infect humans.
Environmental Reservoirs:
- Often challenging to eliminate (e.g., Clostridium).

Portals of Exit: Routes for pathogens in the body (e.g., feces, saliva, skin).
Portals of Entry: Routes through which pathogens infect hosts (e.g., respiratory tract, broken skin).

Vertical Transmission: From mother to fetus/infant.
Horizontal Transmission: Person-to-person.
- Direct Contact: Handshakes, intimacy.
- Indirect Contact: Fomites (inanimate objects).
- Droplet Transmission: Respiratory droplets; controlled through hygiene.
- Food and Water Contamination: Hazards exist in contaminated food and municipal water.
- Airborne Transmission: Smaller particles can carry pathogens longer distances.
- Vectors: Living organisms that carry pathogens (e.g., mosquitoes, ticks).

Virulence: Ability to cause disease; determined by adherence and immune evasion capabilities.
Dose: Minimum quantity of pathogens; extremely low doses may not cause disease.
Incubation Period: Time taken for symptoms to appear; influences spread potential (e.g., typhoid fever spread).

Immunity: Previous exposure/immunization status.
- Herd Immunity: Protects non-immune individuals; often needs >90% immunity.
General Health: Malnutrition, overcrowding influence susceptibility.
Age and Gender: Very young and elderly are more susceptible; specific diseases target certain genders.
Cultural Practices: Dietary habits can affect exposure; breastfeeding protects infants.
Genetic Background: Variation in natural immunity due to genetic differences.

Historical Context: John Snow's cholera investigations laid foundations for modern epidemiology.
Study Types:
- Descriptive Studies: Data collection post-outbreak (focus on person, place, time).
- Analytical Studies: Investigate relevancy of risk factors; includes cross-sectional, retrospective, and prospective studies.
- Experimental Studies: Assess cause/effect relationships, often using control groups.