Principles of Epidemiology in Public Health Practice: An Introduction to Applied Epidemiology and Biostatistics (34 mins)
Scroll down to "Lesson One: Introduction to Epidemiology" and read the section titled "Concepts of Disease Occurence" (pages 1-52 to 1-55), and complete 'Exercise' 1.8 on page 1-56.
Click https://web.archive.org/web/20230519060949/https://www.cdc.gov/csels/dsepd/ss1978/SS1978.pdf link to open resource.