A fisherman casts his net into murky water and pulls out fish. That's roughly how Episcanner works: it sifts through 14 years of disease records and hauls out every fever outbreak in every municipality in Brazil. From 2010 to 2023, the net caught 18,567 dengue epidemics and 2,199 chikungunya epidemics. Previously, such data were collected piecemeal, making it hard to compare north and south. Now all the fish are in one boat.

Each epidemic gets a passport: how many people one patient infects, when the peak hits, how long the illness lasts. For dengue, the average epidemic lasts 26.6 weeks, which is more than half a year. Chikungunya is a bit shorter at 25.5 weeks. But those are national averages; the real picture is as varied as fish scales.

Why is that? Epidemics are spread by mosquitoes, whose activity depends on temperature and rainfall. When the ocean is warm (El Niño), epidemics arrive earlier. When the ocean is cool (La Niña), they come later. A warm ocean seems to spur mosquitoes on.

Now health authorities have an open dataset, and they can plan where and when to set mosquito traps or stockpile medicines. The fisherman isn't just showing off his catch; he's pointing out where in the river to cast the net next time.