Despite a dismal rate of testing, Brazil has the most Covid 19 infections in Latin America and, experts fear, the country may be on the brink of exploding. Especially in the densely populated favelas, or slums, conditions are ripe for the rapid spread of the pandemic. Mapping the virus’s spread is considered crucial to containing it, but without government action, one small project is offering free tests in the country’s biggest favela with worrisome initial results.