1.4 Key concepts and terminology in epidemiology

Epidemiology is all about understanding how diseases spread and affect populations. This section dives into key terms like incidence, prevalence, and risk, which help measure disease occurrence and impact. It's like learning the ABCs of tracking health issues.

The notes also cover study designs, from observational to descriptive. These are the tools epidemiologists use to investigate disease patterns and causes. It's like having different lenses to view health problems, each with its own strengths and weaknesses.

Key Epidemiological Terms

Incidence, Prevalence, and Incidence Rates

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• Incidence refers to the number of new cases of a disease or condition that occur in a specified population over a given period of time, measuring the risk of developing a disease
• Prevalence refers to the total number of cases of a disease or condition that exist in a specified population at a specific point in time or during a given period, measuring the burden of disease in a population
• Incidence rate is the number of new cases of a disease or condition per population at risk in a given time period, measuring the speed at which new cases occur in a population
• Prevalence proportion is the proportion of a population that has a disease or condition at a specific point in time, measuring the frequency of disease in a population

Risk, Odds, and Incidence Proportion

• Risk is the probability or likelihood of an event occurring, such as developing a disease or experiencing an adverse health outcome, often expressed as a percentage or proportion
• Odds is the ratio of the probability of an event occurring to the probability of it not occurring, another way of expressing the likelihood of an event
• Incidence proportion (cumulative incidence) is the proportion of a population at risk that develops a disease over a specified period of time, measuring disease occurrence

Association, Causation, and Confounding

Association and Causation

• Association refers to the relationship between two variables, such as an exposure and an outcome, describing the degree to which the variables are related statistically
  • A positive association means that as one variable increases, the other variable also increases, while a negative association means that as one variable increases, the other variable decreases
  • Association does not necessarily imply causation, as other factors may be responsible for the observed relationship
• Causation refers to the direct relationship between an exposure and an outcome, where the exposure is responsible for causing the outcome
  • Establishing causation requires meeting criteria such as temporality (exposure must precede outcome), strength of association, consistency of findings, biological plausibility, and experimental evidence

Confounding

• Confounding is a situation in which the relationship between an exposure and an outcome is distorted by the presence of another variable that is associated with both the exposure and the outcome
  • Confounding can lead to spurious associations or mask true associations between variables
  • Controlling for confounding variables through study design (randomization, matching) or statistical analysis (stratification, multivariate modeling) is essential to obtain unbiased estimates of the relationship between the exposure and outcome

Epidemiological Study Designs

Observational Studies

• Cohort studies involve following a group of individuals (a cohort) over time to determine the incidence of a disease or outcome in relation to an exposure
  • Strengths: Can establish temporality, calculate incidence rates, examine multiple outcomes, and are useful for studying rare exposures
  • Limitations: May be time-consuming and expensive, subject to loss to follow-up, and not suitable for studying rare outcomes
• Case-control studies involve comparing a group of individuals with a disease or outcome (cases) to a group without the disease or outcome (controls) to determine the association between an exposure and the disease
  • Strengths: Efficient for studying rare diseases, can examine multiple exposures, and require fewer subjects than cohort studies
  • Limitations: Prone to selection and recall bias, cannot calculate incidence rates, and may have difficulty establishing temporality

Descriptive Studies

• Cross-sectional studies involve assessing the prevalence of a disease or outcome and the exposure status of individuals at a single point in time
  • Strengths: Quick and inexpensive, can estimate prevalence, and are useful for generating hypotheses
  • Limitations: Cannot establish temporality, prone to selection and information bias, and may not be suitable for studying rare diseases or exposures
• Ecological studies involve comparing disease rates and exposure levels across different populations or groups, rather than individuals
  • Strengths: Can study large populations, use existing data, and are useful for generating hypotheses
  • Limitations: Prone to ecological fallacy (drawing individual-level conclusions from group-level data), cannot control for individual-level confounders, and may have difficulty establishing causality

Disease Transmission and Natural History

Modes of Disease Transmission

• Direct transmission involves direct contact between the source and the host, such as through skin-to-skin contact, sexual contact, or contact with bodily fluids (blood, saliva)
• Indirect transmission involves the spread of the infectious agent through a vehicle, such as contaminated food, water, or fomites (doorknobs, toys)
• Airborne transmission involves the spread of the infectious agent through the air, such as through droplets (influenza) or aerosols (measles)
• Vector-borne transmission involves the spread of the infectious agent through a living organism (vector), such as mosquitoes (malaria) or ticks (Lyme disease)

Stages of Disease Progression

• Exposure is the initial contact between the host and the infectious agent or risk factor
• Incubation period is the time between exposure and the onset of symptoms, during which the agent is replicating but not causing symptoms
• Subclinical disease is the stage where the disease is present but not causing noticeable symptoms, and some individuals may recover during this stage without ever developing overt symptoms
• Clinical disease is the stage where the disease causes noticeable signs and symptoms, which may be further divided into acute, subacute, and chronic phases, depending on the duration and severity of the disease
• Resolution is the stage where the disease either resolves completely (recovery), transitions into a chronic state, or results in death