Epidemiology Lecture Notes

Epidemiology: Tracking Diseases in Populations

Epidemiology is the study of how diseases move through populations and who gets sick, focusing on the how, when, where, and who of disease.

• It involves diverse backgrounds, including biology, math, psychology, ecology, and sociology, because understanding both the disease and the affected people is crucial.
• It primarily tracks communicable diseases but can also study non-communicable ones.

Chain of Infection

Communicable diseases spread through a chain of infection, involving:

• Entry into a host.
• Presentation within the host.
• Host's compromised status.
• Exit from the host.
• Entry into the next host.

This chain offers multiple points for intervention to break the cycle.

• Improved sanitation.
• Treatment of the sick.
• Vaccination.

Principles of Epidemiology: Rates Over Absolute Numbers

Epidemiology focuses on the rate of disease, which is the frequency of sickness relative to the population, rather than absolute case numbers.

• Attack Rate: Measures how much of the population gets sick.
• Morbidity: Refers to being sick with a disease.
• Mortality: Refers to death due to a disease.

These are often expressed per 100,000 people because percentages may not be usable due to small numbers.

• High Morbidity Example: The flu, due to its contagiousness.
• High Mortality Example: Heart disease, cancer, and dementia in the U.S.

Case Fatality Rate

Case fatality is the proportion of people who die from a specific disease among those infected.

Diseases with a high case fatality rate, such as Ebola or untreated plague, are particularly concerning. Treatment improvements can lower case fatality rates, as seen with AIDS.

Visual Representation

Graphs can illustrate morbidity and mortality rates, such as those for pneumococcal pneumonia, showing the percentage of patients sick or dying from different types of pneumonia.

Incidence vs. Prevalence

• Incidence Rate: The number of new cases in a population over a specific time.
• Prevalence: The total number of cases at a given time.

Example: Disease X

• Early 1980s-1990s: High incidence (new cases) due to spread.
• Later Years: Incidence decreases due to new information on disease spread and treatment.

Prevalence increases as people live longer with the disease, highlighting the importance of monitoring both new and total cases.

Endemic, Epidemic, Outbreak, and Pandemic

• Endemic: A disease constantly present in a population with expected levels of cases (e.g., seasonal flu).
• Epidemic: An unusually large number of cases, either from an endemic disease or a new disease.
• Outbreak: A number of cases in a specific time or group, fairly contained.
• Pandemic: A global epidemic affecting multiple places worldwide (e.g., COVID-19).

Example: Deaths from influenza and pneumonia fluctuate seasonally but can exceed an epidemic threshold, indicating an epidemic.

Reservoirs of Pathogens

A reservoir is the natural habitat where a pathogen lives.

• Environmental: Soil or water.
• Animal: Human or non-human animals.

Human Reservoirs

Humans carry pathogens, knowingly or unknowingly.

• Some diseases are exclusive to humans, making humans the natural habitat for the pathogen.
• Asymptomatic carriers can unknowingly spread diseases.

Non-Human Animal Reservoirs

Animals, like bats (rabies) or rodents (plague), serve as reservoirs.

• Humans are not the natural habitat for these diseases, but transmission is possible.

Environmental Reservoirs

Soil and water can harbor pathogens (e.g. Cholera).

• Eliminating these is challenging, requiring careful protection measures.

Leaving the Reservoir

Exit Strategies for Pathogens:

• Intestinal Tract: Fecal matter.
• Respiratory Tract: Sneezing, coughing (saliva, mucus).
• Skin: Shedding of skin cells.
• Genital Pathogens: Sexual secretions or semen.

Transmission of Disease

Horizontal vs. Vertical Transmission

• Horizontal Transmission: Person to person.
• Vertical Transmission: From pregnant person to fetus, during childbirth, or breastfeeding.

Example: A cold being passed a child would be horizontal transmission. Passing a pathogen to child in the womb would be vertical.

Horizontal Transmission Methods

• Contact: Direct, indirect, or droplet.

  • Direct Contact: Physical contact (e.g., shaking hands).
  • Indirect Contact: Via inanimate objects (fomites) like doorknobs.
  • Droplet Contact: Through respiratory droplets within 3-6 feet.

• Vehicle Transmission: Via contaminated food or water.

  • Can cause large disease spikes, especially with water contamination.

• Air Transmission: Through dried droplet nuclei (microbes in air).

  • Hard to control; common in respiratory diseases.
  • HEPA filters help reduce air transmission in hospitals.

• Vectors: Living organisms (arthropods) carrying pathogens.

  • Mechanical: Pathogen on the outside (e.g., fly).

  • Biological: Pathogen inside (e.g., mosquito).

  • Controlling vectors is effective in preventing disease transmission (e.g., malaria).

Pathogenicity and Virulence Factors

• Virulence factors, make disease more problematic.

  • Capsules (Streptococcus pneumoniae protection).
  • Toxins (E. coli O157:H7).

Organisms with multiple virulence factors have a higher chance of causing disease.

Infectious Dose and Incubation Period

• Infectious Dose: Minimum number of pathogens needed to cause sickness.

  • Below the infectious dose, infection may be asymptomatic or eliminated by the immune system.
  • A large dose can cause serious disease even in individuals with normal immune function.

• Incubation Period: Influences disease spread.

  • Short incubation: Symptoms appear quickly, reducing spread.
  • Long incubation: Allows extensive spread before symptoms appear.

Example: 1963 typhoid fever outbreak spread to multiple countries due to long incubation period.

Host Susceptibility

• Immunization: Critical in determining susceptibility.

  • Herd Immunity: Requires high percentage of population immunity (e.g., 96-97% for measles) to prevent disease spread.

• General Health: Malnutrition, overcrowding, and fatigue increase susceptibility.

• Age: Very young and elderly are typically more susceptible due to immune system status.

Example: Tetanus cases are lower in the young (vaccinated) and old, but higher in middle-aged adults who may not have had boosters.

Case Study: John Snow and the Cholera Outbreak

John Snow tracked a cholera outbreak in London in the mid-1800s by:

• Collecting data on who was sick, where they were sick, age, gender and socioeconomic class.
• Mapping cases of cholera to identify the source.
• Identifying the Broad Street well as the likely source.
• Removing the well handle, stopping the outbreak and also convincing people something needed to be done.

This descriptive study demonstrated how identifying who gets sick, where, and when can curb disease spread.

Descriptive Studies: Patterns in Disease

• Seasonality: Diseases can vary by season (e.g., campylobacter in summer, RSV in winter).

• Exposure: Can be common-source or propagated.

  • Common Source: Everyone exposed at one source.
  • Propagated: Host to Host

National Disease Surveillance Network

• The CDC's National Disease Surveillance Network monitors diseases.

• Doctors, clinicians, hospitals, and schools are required to report notifiable diseases immediately.

  • Diphtheria
  • E. coli O157:H7
  • Plague
  • Polio
  • Smallpox