Two experiments for Teaching Immunity with a Classroom Epidemic Simulation

Immunity is one of those biology topics that can feel a bit abstract to students. We talk about pathogens, antigens, and antibodies, but unless they’ve actually been ill (or recently jabbed), the concepts don’t always stick. That’s where a classroom epidemic simulation comes in — a hands-on way to show how infections spread and how immunity protects us.

The Simulation Setup

You don’t need anything fancy to run this. A simple version uses test tubes or cups of clear liquid: most contain water, but a few secretly contain sodium hydroxide solution (or another safe indicator-ready liquid). Students “interact” by exchanging a few drops with each other using pipettes. After several rounds, you add a few drops of phenolphthalein indicator — and suddenly some test tubes turn pink.

That’s your epidemic!

The original “infected” test tubes show who the first cases were.
The chain of pink test tubes shows how disease spreads through contact networks.
Students quickly realise that one or two interactions can spread the “disease” to the whole class.

Linking to Immunity

Once students have seen the spread, you can introduce the immune system’s role:

Innate immunity – our first line of defence (skin, phagocytes).
Adaptive immunity – specific responses, where B-cells produce antibodies to match antigens.
Memory cells – why a second infection is usually defeated much faster.

You can even repeat the simulation with a twist: a few students are “vaccinated” and refuse exchanges. Suddenly, the “disease” spreads much less effectively — a perfect way to demonstrate herd immunity.

Why It Works

This activity makes abstract biology real. Instead of memorising terms, students experience the spread of infection and see the importance of immunity in stopping it. It sparks discussion, encourages critical thinking, and works brilliantly at both GCSE and A-Level (with more detail on antigen-antibody specificity for the older students).

Extension Experiment: “Tokens in Spheres” – How Immunity Ends an Outbreak

This tabletop simulation models how an infectious disease grows, peaks, and fades as natural immunity builds in a population.

Materials

26 opaque plastic spheres/capsules (or ping-pong balls with stickers).
26 small plastic counters (“tokens”), one per sphere.
1 cloth bag (opaque).
Whiteboard or grid paper to draw a bar chart by round.
Marker and simple results table.

Meaning: each sphere = one person. A token inside = still susceptible. Removing the token = infected then immune.

Setup

Put one token inside each sphere and all 26 spheres into the bag.
Draw a results table with columns:
Round | Draws | New Cases | Cumulative Cases | Susceptible Left | Immune.

Rules of Play

Round 1: Draw 1 sphere from the bag.
- If it contains a token (it will, at the start), that’s 1 new case. Remove the token (the person becomes immune) and return the sphere to the bag.
- Plot the bar for Round 1 (height = new cases).
Next rounds: The number of draws = 2 × (new cases from the previous round).
- For each sphere you draw: if it still has a token, that’s a new case; remove the token and return the sphere. If it has no token, they’re already immune—no new case—return it.
- Tally new cases, update the table, and plot the bar.

Continue until a round produces 0 new cases (the outbreak has died out).

The multiplier “2” is your classroom R (each case seeds two exposure attempts next round). You can change it to explore different R values.

What Students See

Early rounds: bars rise (exponential-like growth).
Middle: the peak—lots of draws are “wasted” on people who’ve already become immune.
Late: bars fall to zero—herd effects emerge as the susceptible pool shrinks.

Sample Results Table (blank to copy)


Round | Draws | New Cases | Cumulative Cases | Susceptible Left | Immune
------|-------|-----------|------------------|------------------|-------
  1   |   1   |           |                  |                  |
  2   | 2×R1  |           |                  |                  |
  3   | 2×R2  |           |                  |                  |
  …   | 2×R…  |           |                  |                  |

Debrief Questions

Why do new cases peak even though we keep drawing more spheres at first?
What does removing tokens represent biologically?
How does changing the multiplier from 2 to 1.5 or 3 alter the curve?
Where would you place the herd immunity threshold in this model?

GCSE Links

Communicable disease, immune response, vaccination, herd immunity (qualitative).
Reading simple bar charts; relating shape to mechanism.

A-Level Links

S-I-R ideas (Susceptible → Infected → Removed/Immune).
R₀ and effective reproduction number Rₜ = R₀ × (S/N).
As S falls, Rₜ < 1, so cases decline.
Stochastic effects: different runs give slightly different peaks.

Variations

Vaccination start: Pre-remove tokens from 20–40% of spheres before Round 1. Compare peak height and timing.
Different R: Use 1.2, 1.5, 3.0 by changing the next-round draw rule (e.g., Draws = round(R × previous cases)).
Limited mixing: Cap the max draws per round to model behaviour change.
Reinfection window (advanced): After 3 rounds, allow 10% of immune spheres to “regain” a token to discuss waning immunity (clearly label as a what-if).

Safety & Practical Notes

Keep tokens large enough to avoid choking hazards.
Use a sturdy opaque bag so students can’t see inside.
If you’re short on time, run with 13 spheres and the same rules.

✅ With two simple classroom experiments, immunity becomes more than a textbook definition — it becomes a lived demonstration of why our immune systems (and vaccines) matter so much. At Hemel Private Tuition, we believe that experiments create memorable moments that stay with the students to help them understand.

Download the free worksheet at the bottom of the page

Monday, 1 September 2025

Teaching Immunity with a Classroom Epidemic Simulation