Dengue in Costa Rica is like a sea tide: it comes regularly, but the wave height varies each time. In some cantons the disease flares up every year, in others every two or three years. Twenty years of observations show that these waves are not random: they have a rhythm that you can hear if you break the data into waves of different lengths, like music into notes.

Scientists took monthly incidence data from 32 cantons and compared them with what is visible from space: how green the foliage is, how moist it is, and what the surface temperature is during the day and at night. It turned out that on the Pacific coast, the fever rises three months after the foliage becomes denser. Surface temperature and the warm El Niño current are linked to the disease nationwide, but with a delay: the dengue peak comes 3–6 months after the coldest moment.

Now it is clear why dengue behaves differently in different places. On the coast, the main signal comes from foliage, while inland it comes from the ocean. So preparations for outbreaks must also differ: in some areas watch satellite images of vegetation, in others watch ocean temperature. This way the fever stops being unpredictable: its rhythm can be heard in advance.