Volunteers in Salvador’s favelas are collecting data on deadly infections and inequality to help campaign for better sanitation

Wearing crisp, white T-shirts and carrying tablets, the students fan out through Marechal Rondon – a bustling favela spread over hillsides and a valley in Brazil’s north-eastern city of Salvador. As they walk, they map blocked drains and piles of rubbish on their tablets. These are the “infection points” that attract the rats and mosquitoes which, in turn, spread diseases like leptospirosis and the Zika virus, both prevalent here.

Student Alexandre Santos, 20, stops before a weigh-high tangle of wild plants overlooking a housing block. “We look at sewers, rubble, garbage. Now there is high vegetation,” Santos says, tapping in the data. “It goes straight into the data bank.”