Lesson plan Grade 12 Health & Consumer Science
Tracing the Pellarney Outbreak: Attack Rates and a Tap Water Source
A grade 12 health lesson in which students use survey data to compute attack rates and relative risk, trace a parasite outbreak to tap water, and test a rival claim.
Objectives
- Students will calculate attack rates and relative risks from outbreak survey data and use them to identify the most likely source of illness.
- Students will explain why Cryptosporidium can pass through a chlorinated water system when filtration falters.
- Students will describe the household actions a boil-water advisory requires and identify who faces the greatest risk of severe illness.
Vocabulary
- Turbidity
- Turbidity is a measure of how cloudy or murky water looks because of tiny particles floating in it. Clear water has low turbidity, and muddy water has high turbidity.
- Boil water advisory
- A public notice telling people to boil tap water before drinking it because germs may have entered the pipes, often after a main break or a loss of pressure.
Also: attack rate, relative risk, observational study, Cryptosporidium, oocyst, dose-response
Teacher overview
Health departments rarely identify the source of a waterborne outbreak from a single lab test. They find it by comparing the people who became ill with the people who did not. In this lesson students work through a fictional outbreak in the town of Pellarney, where days of heavy rain muddied the river that feeds the treatment plant and, several days later, clinics began seeing residents with watery diarrhea caused by the parasite Cryptosporidium. Students compute attack rates and relative risks from a household survey, check for a dose-response pattern, test a rival explanation, and then connect the findings to the household behaviors a boil-water advisory asks for.
Two misconceptions are addressed directly. The first is that chlorinated tap water cannot carry a germ. Cryptosporidium is protected by a tough outer shell that lets it survive the chlorine doses used in drinking water, so filtration is the main barrier, and the federal rule that finished water from conventional filtration plants stay at or below 0.3 NTU in 95 percent of monthly samples exists partly for this reason. The second is statistical: students often assume that the group with the most sick people must be the source. A fair comparison uses rates, not raw counts, because the groups being compared are rarely the same size.
Procedure
Hook (5 min): Read aloud this fictional notice: "The Pellarney Water Department and the county health department advise all customers to boil tap water before drinking it, making ice, brushing teeth, or preparing food or infant formula, until further notice." Ask the class: "If you were the county epidemiologist, how would you show that the water caused these illnesses and not a restaurant, the town pool, or a school?" Record ideas on the board without judging them.
Direct instruction (12 min): Introduce Cryptosporidium. It spreads when people swallow something contaminated with human or animal feces, including untreated or poorly treated water. Its outer shell, called an oocyst, resists chlorine at drinking-water doses, so filters, and in some plants ultraviolet light, do the work of removing or inactivating it. Illness brings watery diarrhea and cramps that can last for days or weeks, and it can be severe and hard to treat in people with weakened immune systems. Then define the two tools of the lesson. The attack rate is the number of people who became ill divided by the number of people in the group, written as a percent. The relative risk is the attack rate in the exposed group divided by the attack rate in the unexposed group. A relative risk near 1 means the exposure made little difference, and a relative risk well above 1 points toward the exposure as a cause.
Worked example (8 min): Model the method with a fictional church supper that has nothing to do with water. Sixty guests ate the potato salad and 30 became ill, while 50 guests skipped it and 5 became ill. The attack rates are 30 divided by 60, or 50 percent, and 5 divided by 50, or 10 percent, so the relative risk is 50 divided by 10, which is 5.0. Then show why counts can mislead: if 600 guests had skipped the salad and 50 of them had become ill, the count of 50 would be larger than 30, yet the attack rate among those guests would be only about 8 percent.
Pair work on the handout (25 min): Pairs complete handout items 1 through 12. Circulate and check that item 3 divides two rates rather than two counts, and that item 6 is rounded to two decimal places. When a pair finishes early, ask them what a relative risk of exactly 1 would mean for the town pool.
Formative check and close (10 min): Build the epidemic timeline together on chart paper from the handout's daily case counts, marking the two days of high turbidity. Then give an exit ticket on an index card: "Give one reason a relative risk of 6.0 still does not prove, by itself, that the tap water caused the outbreak." Expected answers mention that an observational study cannot rule out every other shared exposure, or that the conclusion also rests on the turbidity record, the lab findings, the dose-response pattern, and the timing. If fewer than about three-quarters of the cards give a reason of that kind, open the next class by revisiting the difference between an association and proof of cause.
Student Handout
Background. Pellarney is a fictional town that draws its drinking water from the Olvercott River. In our scenario, four days of heavy rain turned the river muddy at the end of April. On May 2 and May 3, the plant's filtered water turbidity rose above the 0.3 NTU standard. The plant kept adding chlorine the whole time. Starting May 6, local clinics reported residents with watery diarrhea, stomach cramps, and nausea, and lab tests found Cryptosporidium, a parasite. Its outer shell, called an oocyst, protects it from chlorine at the levels used in tap water, so the plant's filters are the main barrier that removes it. Swallowing even a small number of these parasites can cause illness. In people with weakened immune systems, such as people receiving cancer treatment or living with advanced HIV, the illness can be severe and hard to treat. In this scenario, the county fact sheet states that symptoms usually begin about a week after a person swallows the parasite.
The county health department interviewed 650 residents about what they drank and did from late April through early May. The fictional survey results are below.
Table A. Tap water
| Group | Residents interviewed | Became ill |
|---|---|---|
| Drank unboiled Pellarney tap water | 400 | 96 |
| Did not drink Pellarney tap water (bottled water or a private well only) | 250 | 10 |
Table B. Glasses of tap water per day (the 400 tap water drinkers only)
| Glasses per day | Residents interviewed | Became ill |
|---|---|---|
| 1 to 2 | 200 | 32 |
| 3 to 4 | 150 | 45 |
| 5 or more | 50 | 19 |
Table C. The town pool
| Group | Residents interviewed | Became ill |
|---|---|---|
| Swam at the Pellarney town pool | 80 | 12 |
| Did not swim at the town pool | 570 | 94 |
Daily count of new cases: May 6: 4, May 7: 7, May 8: 12, May 9: 18, May 10: 24, May 11: 16, May 12: 11, May 13: 8, May 14: 4, May 15: 2.
- Using Table A, what was the attack rate among residents who drank unboiled Pellarney tap water? Give your answer as a percent.
- Using Table A, what was the attack rate among residents who did not drink Pellarney tap water? Give your answer as a percent.
- Using the counts in Table A, calculate the relative risk of illness for tap water drinkers compared with residents who did not drink the tap water.
- Using Table B, what was the attack rate among residents who drank 5 or more glasses of tap water per day? Give your answer as a percent.
- The attack rates in Table B for the 1 to 2 glass group and the 3 to 4 glass group are 16 percent and 30 percent. Describe the pattern across all three groups, including your answer to item 4, and explain why this pattern strengthens the case against the tap water.
- Using Table C, calculate the relative risk of illness for pool swimmers compared with residents who did not swim at the pool. Round to two decimal places.
- A local news post claims that the town pool caused the outbreak. Use your answers to items 3 and 6 to evaluate that claim.
- Investigators did not assign anyone to drink or avoid tap water; they recorded what residents had already done. Explain why the Pellarney investigation had to be an observational study rather than a randomized experiment.
- Filtered water turbidity was above the standard starting on May 2, and the daily case count peaked on May 10. How many days after the first day of high turbidity did the case count peak?
- The plant kept adding chlorine during the entire event. Using the background, explain why chlorine did not prevent this outbreak.
- The advisory tells households to throw away ice made in early May and to brush their teeth with boiled or bottled water. Explain why these uses matter even though no one is drinking a full glass of tap water.
- The advisory tells one group of residents to contact their doctor if they develop diarrhea. Name that group and explain, using the background, why they need extra caution.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
- Answer: 24%.
96 ill divided by 400 interviewed equals 0.24, which is 24 percent. Check: 0.24 times 400 equals 96. - Answer: 4%.
10 ill divided by 250 interviewed equals 0.04, which is 4 percent. Check: 0.04 times 250 equals 10. - Answer: 6.0 (tap water drinkers were six times as likely to become ill).
The attack rate among drinkers, 0.24, divided by the attack rate among non-drinkers, 0.04, equals 6.0. A student who divides the raw counts, 96 by 10, gets 9.6 and has ignored the different group sizes. - Answer: 38%.
19 ill divided by 50 interviewed equals 0.38, which is 38 percent. Check: 0.38 times 50 equals 19. - Answer: The attack rate rises steadily with the amount of tap water people drank, from 16 percent to 30 percent to 38 percent.
This is a dose-response pattern. If the water carried the parasite, drinking more of it should mean swallowing more parasites and a higher chance of illness, which is exactly what the table shows. A cause unrelated to the water would not be expected to line up with glasses per day. - Answer: 0.91.
Swimmers: 12 divided by 80 equals 0.15. Non-swimmers: 94 divided by 570 equals about 0.1649. The relative risk is 0.15 divided by 0.1649, which is about 0.9096, or 0.91. Check a second way: 0.15 times 570 divided by 94 equals 85.5 divided by 94, which is also about 0.91. - Answer: The claim is not supported.
A relative risk of 0.91 is close to 1, so swimmers were no more likely to become ill than non-swimmers, while tap water drinkers had a relative risk of 6.0. Strong answers add that Cryptosporidium can spread in pools, so the pool is a reasonable thing to check, yet in this outbreak the data point to the tap water. - Answer: Investigators could not ethically assign people to drink water suspected of carrying a parasite, and the exposure had already happened, so they could only compare exposures residents had chosen themselves.
That makes the investigation an observational study that gathered its data with a survey. Accept answers that name either the ethical barrier or the timing, as long as the study is correctly identified as observational. - Answer: 8 days.
May 10 minus May 2 equals 8 days. Check: counting forward 8 days from May 2 reaches May 10. The key can note that a peak about a week after the first day of high turbidity fits the scenario's stated delay between swallowing the parasite and becoming ill. - Answer: The oocyst shell protects Cryptosporidium from chlorine at tap water levels, so the plant depended on its filters to remove it.
The high turbidity on May 2 and May 3 shows the filters were not performing well, which means parasites could pass through while the chlorine had little effect on them. - Answer: Ice and toothbrushing water come from the same tap, and swallowing even a small number of parasites can cause illness.
Freezing water does not make it safe, and people swallow some water when they brush their teeth. Accept any answer that connects these uses to swallowing small amounts of contaminated water. - Answer: People with weakened immune systems, such as people receiving cancer treatment or living with advanced HIV.
The background states that the illness can be severe and hard to treat in this group, so early medical advice matters more for them than for most healthy adults.
Differentiation
Support
Give students a blank two-row table for each comparison with the formula printed above it: attack rate equals ill divided by interviewed, times 100. Complete item 1 together as a class before pairs begin. Let pairs answer items 8, 10, 11, and 12 orally to the teacher before writing, so the writing load does not hide what they understand.
Extension
Ask students to compute the relative risk for residents who drank 5 or more glasses per day compared with residents who did not drink the tap water, and then to write a short paragraph on a harder question: several of the 10 ill non-drinkers lived with a tap water drinker who became ill earlier. What does that suggest about how Cryptosporidium can spread, and how should the health department adjust its advice to households with a sick family member?
ELL supports
Pre-teach "attack rate," "relative risk," "advisory," "parasite," and "immune system" with a short glossary that pairs each term with a simple example from the lesson. Read the background aloud while students follow along, pausing after each sentence about the parasite. Provide sentence frames for the written items, such as "The pool claim is not supported because the relative risk for swimmers was ___, while for tap water drinkers it was ___."
Extensions
Have students find their own water utility's most recent annual water quality report and locate the section on turbidity or on Cryptosporidium monitoring, if the report includes one. Students can write a short explanation, suitable for a neighbor, of what the utility measures and why turbidity is watched so closely at a plant that also uses chlorine.