RIO DE JANEIRO—In early May, the new coronavirus swept through the crowded homes in Alley 24 of Rocinha, one of the largest of Brazil’s favelas, or slum communities.

Twenty of the 35 members of Ivanete Dias de Carvalho’s extended family who live crammed together here came down with symptoms of Covid-19, from high fevers to fluid-filled lungs. Few were able to get a test. Her 65-year-old aunt died.

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