Situation: An earthquake damaged houses and the piped water supply of a municipality. Six hundred families, averaging 5 members each, stay in a school building used as an evacuation center, where the public health nurse leads the health team. The nurse's surveillance log of new cases of watery diarrhea in the center shows: Day 3: 2; Day 4: 9; Day 5: 14; Day 6: 15; Day 7: 13; Day 8: 16; Day 9: 14 All cases drank from one storage tank refilled daily from a damaged municipal line. The usual incubation period is 1 to 3 days. Which pattern do these data show?

Cases rose quickly and then stayed on a plateau for longer than one incubation period, which is the shape of a continuous common-source outbreak. A tank refilled daily from a damaged line keeps exposing evacuees, so treating or replacing the water supply is urgent. A point source would peak once and decline within about one incubation period, and a propagated outbreak would show successive, taller waves.

The daily case counts form a distinct curve: 2, 9, 14, 15, 13, 16, 14. After the initial rise, cases do not fall back to baseline. Instead, they remain elevated across multiple days. This sustained plateau is the epidemiologic signature of a continuous common-source outbreak.

Why the plateau matters

In a point-source outbreak, everyone is exposed during a narrow window—often a single meal or event. Cases then appear within one incubation period and decline quickly once the source is removed or consumed. The retrospective cohort study of a religious ceremony in Bangladesh illustrates this pattern: all cases ate food served at one event on a single day, so the outbreak rose and fell around that exposure ^[1]. Similarly, the polymicrobial foodborne outbreak in Southern Italy was traced to a point source, meaning the exposure did not continue over time ^[3].

Here, the incubation period is 1 to 3 days. If the contaminated water exposure had stopped after day 3, new cases should have tapered off by roughly day 6. Instead, new cases continued at 13 to 16 per day through day 9. A plateau lasting longer than one incubation period indicates that people are being exposed repeatedly, not just once.

Linking the tank to continuous exposure

The storage tank is refilled daily from a damaged municipal line. Each refill can introduce new contamination, so every day brings a fresh opportunity for exposure. This matches the definition of a continuous common source: a single vehicle—the tank water—keeps delivering the pathogen over an extended period. The school-based milk outbreak in Toyama, Japan, followed a similar logic: a contaminated product distributed repeatedly across multiple schools produced widespread, sustained cases until the source was controlled ^[2].

Key point! A continuous common-source outbreak does not require person-to-person spread. The ongoing exposure comes from the environment or vehicle, not from infected individuals transmitting to others.

Ruling out the other patterns

Pattern	Expected curve	Fits this data?
Point-source	Sharp rise, single peak, decline within one incubation period	No—plateau persists beyond day 6
Continuous common-source	Rapid rise, then sustained plateau while exposure continues	Yes—matches days 3 through 9
Propagated	Successive, progressively taller waves separated by incubation intervals	No—no wave-like peaks
Sporadic	Scattered cases with no clear link or temporal clustering	No—cases cluster tightly in time and share a common water source

A propagated outbreak would show one wave generating a second, larger wave as person-to-person transmission takes hold. The group C rotavirus outbreak in a primary school in Luzhou, China, demonstrated spread across multiple grades, consistent with person-to-person or grade-level transmission rather than a single shared vehicle . The data here show no such wave pattern.

Clinical and public health response

The priority is to stop the ongoing exposure by treating or replacing the water supply. While individual case management—oral rehydration, monitoring for dehydration, and infection control precautions—remains essential, it will not end the outbreak if the tank continues delivering contaminated water. The investigation should also confirm the pathogen through stool cultures or molecular testing, trace the exact point of contamination in the damaged line, and assess whether bottled water or point-of-use disinfection can serve as an interim measure.

Watch out! Do not mistake a sustained plateau for a propagated outbreak simply because cases remain high. The shape of the epidemic curve—not the total case count—distinguishes continuous common-source from person-to-person spread.