One outbreak, start to finish. You're the field investigator: write the case definition, find the cases, chart the curve, name a suspect, run the numbers, and close the case. Dr. Osei checks your work at every step.
Since Monday, Cupertino-area clinics and the ER have logged 14 reports of acute watery diarrhea this week, most in kids ages 5–13. Several mention the same place: Sunset Aquatic Center, the county pool. That's well above what we'd normally expect.
I need you to run this one start to finish: confirm it's real, define a case, find every case you can, chart it, name a suspect, test your hypothesis with real numbers, prove it, and write the plan to shut it down. I'll be checking your work at each step.
Dr. Osei
Step 2 of the 10-step checklist: establish that an outbreak actually exists
Before you treat this as an outbreak, Dr. Osei wants to know you'd check the right thing first.
Step 4: person, place & time, the foundation everything else is built on
Patients report watery diarrhea, several with cramps or a mild fever, though a few report just one loose stool and nothing else. Sunset Aquatic Center is open all summer as usual, but a lifeguard's log shows a fecal incident in the main pool on July 5, handled with a routine chlorine shock the same day. Reported onset dates so far run from July 8 to July 27.
Who counts as a case, symptom-wise?
Which place criterion actually matches the evidence?
Which onset window fits the July 6–17 exposure period and a plausible incubation range?
Step 4, continued: find cases systematically, applying your own case definition to real callers
Eight calls came in overnight. Using the case definition you just wrote, check every caller who counts as a case.
| Caller | Age | Visited? | Onset | Symptoms | Case? |
|---|
Step 5: descriptive epidemiology, and what the curve's shape tells you
The full case list (71 cases so far) plotted by date of symptom onset:
This wide, gradually rising-and-falling shape most likely reflects:
Step 6: name the most likely agent, given everything you've collected
Attack rate by area used, among the 227 visitors interviewed so far (all 71 cases, plus visitors who stayed well):
| Area used | Attendees | Ill | Attack rate |
|---|---|---|---|
| Main pool | 83 | 54 | 65% |
| Splash pad only | 84 | 14 | 17% |
| Concession stand only (never in water) | 60 | 3 | 5% |
Symptoms are watery, non-bloody diarrhea, sometimes with a low-grade fever. Stool samples show no blood. Pick the suspect whose profile fits every clue, including that July 5 chlorine shock that didn't stop the spread.
Step 7: evaluate the hypothesis with a retrospective cohort of a known group
Kids Club Day Camp visited Sunset Aquatic Center as a group on July 10. Because you have the full roster, you can run a cohort study directly:
| Ill | Well | Total | |
|---|---|---|---|
| Swam in main pool | 42 | 18 | 60 |
| Splash pad only | 8 | 72 | 80 |
| Total | 50 | 90 | 140 |
And what does that RR actually mean?
Match each piece of evidence to the causation criterion it demonstrates
Step 9: control & prevention, acting on what you found
Select every action that belongs in the control plan:
Now tag three already-taken actions by their level of prevention:
Confirmed cause: Cryptosporidium, transmitted through the main pool at Sunset Aquatic Center: a continuous common-source outbreak. A fecal incident on July 5 was cleaned with a routine chlorine shock, but Cryptosporidium oocysts tolerate normal chlorine levels for days, so the facility kept exposing new visitors until it closed on July 17.
Evidence: in the Kids Club cohort, attack rate 70% among main-pool swimmers vs. 10% splash-pad-only (RR = 7.0), echoing the 65% vs. 17% split among all interviewed visitors; onset consistently 5–9 days after individual exposure, matching Cryptosporidium's incubation period; five other pools statewide reported linked clusters this summer.
Control measures: pool closed and hyperchlorinated per CDC fecal-incident protocol; symptomatic swimmers excluded until 2 weeks symptom-free; past visitors and local clinicians notified.
Want another pass, or a refresher on any piece of this before your next case?