In the Brazilian city of Recife, three viruses (dengue, Zika, and chikungunya) are spread by mosquitoes. Scientists collected data on thousands of cases and built a model that assesses how infections in different parts of the city influence each other. The model looks at two things: how close cases are in space and how much time passes between them.

Time, however, turned out to be key. If you consider only short intervals, the three diseases look different, but that difference is deceptive: it arises simply because cases coincide in time. When the model was restricted to biologically plausible time frames (for example, accounting for how long a mosquito lives and how long the virus takes to mature inside it), a common transmission structure emerged for all three diseases. Differences between dengue, Zika, and chikungunya appear only when you look at delays longer than a critical window.

This approach is like tuning a radio: on short waves you hear only static, but once you set the right band, a signal appears. The reconstructed transmission networks show local hotspots, consistent with the real epidemic picture. The takeaway is simple: to tell one epidemic from another, don't look at the map — look at the clock.