Endemic Diseases

When immunity doesn’t last

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Without lasting immunity (SIS), infections can become endemic.
• Endemic level $I^* = 1 - 1/R_0$.
• Diseases persist when new susceptible people keep arriving.

Steps

1. The SIS model
2. The steady state
3. Set the endemic level
4. Why some diseases never leave
5. How long are you ill?
6. Halving the burden