The Epidemiology Bathtub: A Surprisingly Simple Way to Understand Disease Spread
I’ll be honest — when I first started learning about epidemiology, I struggled. Trying to wrap my head around concepts like incidence, prevalence, and recovery rates felt like learning a new language. Then someone told me to imagine a bathtub, and suddenly, everything made sense.
Yes, a bathtub.
It sounds weird, right? But this simple mental model explains how diseases move through a population better than any textbook diagram I’ve seen. Let me walk you through it.
The Bathtub: The Population
Imagine a regular old bathtub. The water inside it represents people in a population — both healthy and sick. The water level rises and falls depending on how many people are getting sick or recovering.
I remember picturing this in my head during a lecture, and it was like a lightbulb moment. It turned all those abstract concepts into something visual, something I could grasp.
The Faucet: New Infections (Incidence)
The faucet dripping water into the tub represents incidence — the number of new cases popping up. The faster the faucet flows, the more people are getting infected.
For example, during COVID-19’s early days, that faucet was on full blast. People were getting infected rapidly, and the water level (prevalence) shot up. But when public health measures kicked in — masking, social distancing, vaccines — the faucet slowed down.
I used to picture this faucet when reading news headlines. It made it easier to understand what was happening without getting lost in the numbers.
The Water Level: Total Cases (Prevalence)
The water level in the tub represents prevalence — the total number of people currently sick. If more water flows in than drains out, the level rises. If the drain works faster than the faucet, the level drops.
It’s like watching a community grapple with an outbreak:
• High water level: A disease spreading uncontrollably.
• Stable water level: A disease that’s endemic but manageable.
• Low water level: A disease on its way out.
I remember thinking about this during the tail end of the COVID pandemic. Cases were dropping, but the water level never hit zero. It made me realize how hard it is to completely “drain” some diseases from a population.
The Drain: Recovery or Death
The drain at the bottom of the tub represents people leaving the sick population — either by recovering or dying. A clogged drain means people stay sick longer, while a wide-open drain means they recover (or pass away) quickly.
It sounds a bit harsh, I know. But this part of the model helps explain why treatments are so important. For diseases like the flu, medications can “widen the drain,” helping people recover faster and lowering the overall water level.
During my public health officer days, I saw this firsthand. Access to treatments didn’t just help individuals — it relieved pressure on entire communities.
Why This Model Is So Useful
The beauty of the bathtub model is that it shows where we can intervene:
• Turn down the faucet: Prevent new infections with vaccines, sanitation, and education.
• Widen the drain: Speed up recovery with better treatments and healthcare access.
• Monitor the water level: Track prevalence to catch outbreaks early.
I remember learning about smallpox eradication and thinking, “They basically turned off the faucet.” It made me appreciate the power of public health on a whole new level.
Final Thoughts
Epidemiology can be complex, but the bathtub model strips it down to the basics. It’s not perfect, but it’s a great starting point for understanding how diseases move through a population.
Next time you hear about an outbreak, just picture that bathtub. Is the faucet running wild? Is the drain clogged? Those simple mental images can make a world of difference in understanding what’s happening — and what can be done to stop it.
And honestly, anything that makes epidemiology feel less intimidating is a win in my book.
