1. Introduction

In epidemiology, we don’t just ask who is sick; we ask how many and how fast. Measuring disease frequency is the foundation for allocating resources, identifying outbreaks, and evaluating interventions.

Key Distinction: Counts vs. Proportions vs. Rates

  • Count: The absolute number of cases (e.g., 50 people have flu).
  • Proportion: A ratio where the numerator is part of the denominator (e.g., 50/1000 = 5%).
  • Rate: A measure of how quickly cases occur, incorporating time in the denominator.

2. Measures of Disease Frequency

A. Prevalence

Prevalence measures the burden of disease in a population at a specific time. It includes both new and pre-existing cases.

Formula: \[Prevalence = \frac{\text{Total number of cases at a given time}}{\text{Total population at that time}}\]

  • Point Prevalence: A snapshot at a single moment (e.g., “Cases on Dec 29”).
  • Period Prevalence: Cases over a window (e.g., “Cases during 2025”).

B. Incidence

Incidence measures the flow of new cases. It represents the risk of contracting the disease.

1. Cumulative Incidence (Risk): \[CI = \frac{\text{Number of NEW cases during a period}}{\text{Number of persons at risk at the start of the period}}\]

2. Incidence Rate (Incidence Density): Used when people are followed for different lengths of time. \[IR = \frac{\text{Number of NEW cases}}{\text{Total Person-Time at risk}}\]

The “Bathtub” Analogy: Imagine a bathtub. The water flowing in is Incidence. The water already in the tub is Prevalence. The water draining out (death or cure) is the Outcome.

3. Real-Life Example: COVID-19 & Diabetes

In 2020-2023, epidemiologists used these measures to identify vulnerable groups.

  • Prevalence Example: A study in New York found that among hospitalized COVID-19 patients, the prevalence of diabetes was approximately 34.6%. This helped hospitals prepare for specialized metabolic care.
  • Incidence Example: During the Omicron wave, the daily incidence rate spiked to over 200 cases per 100,000 person-days in some regions, signaling the need for immediate lockdowns.

4. Measurement of Mortality

Case Fatality Rate (CFR)

CFR measures the severity of a disease—the proportion of people diagnosed who eventually die from it.

Formula: \[CFR (\%) = \frac{\text{Number of deaths from disease X}}{\text{Number of confirmed cases of disease X}} \times 100\]

  • Real-World Data: The CFR for COVID-19 varied significantly by age. For those <40, it was often <0.1%, but for those >80, it exceeded 15% in early 2020.

5. Visualizing Disease: The Epi Curve

Epidemiologists use Epidemic Curves (Epi Curves) to visualize the progression of an outbreak.

Figure 1: Sample Epidemic Curve showing the progression of new cases over 30 days.

Figure 1: Sample Epidemic Curve showing the progression of new cases over 30 days.

6. Summary Table

Measure Numerator Denominator Application
Prevalence All cases (Old + New) Total Population Planning health services
Incidence New cases only Population at Risk Identifying causes/outbreaks
CFR Deaths from disease Confirmed cases Measuring virulence/severity

7. Discussion Questions

  1. If a new drug successfully keeps patients alive longer but does not cure them, what happens to the Prevalence and Incidence of the disease?
  2. Why is the “Population at Risk” used as the denominator for Incidence but not necessarily for Point Prevalence?

```