Epidemiology Basics: Incidence, Prevalence, Study Designs and Outbreaks
In short: Epidemiology studies how disease is distributed in populations and what determines it. Key measures are incidence and prevalence; key designs are cohort, case-control, cross-sectional and randomised trials.
Definition
Epidemiology is the study of the distribution and determinants of health-related states in specified populations, and the application of this study to control health problems. It answers who, what, where, when and why.
Incidence and prevalence
Incidence counts new cases in a period divided by the population at risk. Prevalence counts all existing cases at one time. A chronic disease like diabetes can have low incidence but high prevalence because people live with it for years.
Study designs
Descriptive studies describe patterns. Cross-sectional surveys, such as Nepal's Demographic and Health Survey, measure exposure and outcome at one time. Case-control studies compare people with and without a disease. Cohort studies follow exposed and unexposed groups over time — the British Doctors Study by Doll and Hill linked smoking to lung cancer. Randomised controlled trials give the strongest evidence for interventions.
Outbreak investigation
Standard steps: confirm the outbreak and diagnosis, define a case, find and count cases, describe by time, place and person (epidemic curve), form and test hypotheses, implement control measures, and communicate findings.
Key takeaways
- Incidence = new cases; prevalence = all existing cases.
- RCTs give the strongest evidence for interventions.
- Epidemic curves show how an outbreak develops over time.
Common questions
What is the difference between an epidemic and a pandemic?
An epidemic is more cases than expected in an area; a pandemic is an epidemic spread across countries or continents.
Where can I practise calculations?
Use PHC-Nepal's Epi & Health Tools page for rates, sample size and more.
References
Educational content by PHC-Nepal. It does not replace advice from a qualified health professional.