HAVANA — Silently and without pause, the giant African snail has been invading Cuba. With their shiny, brilliantly striped shells and bodies up to 8 inches long, the snails have become public enemy No. 1 for epidemiologists, and citizens have grown to fear their ability to transmit diseases and harm crops. ‘‘I never heard of them before, but now they’re everywhere,’’ said Yusmila Marín, a 29-year-old nurse who lives in a neighborhood rife with the mollusks, whose scientific name is Achatina fulica. In recent months, Cuban authorities have activated the nationwide civil defense system to battle the snails. But some say the state’s response is inadequate. Advertisement Marín and families in the Villa Panamericana neighborhood are keeping children indoors to play due to the danger of infection from the snails, which can transmit a potentially severe brain infection called meningoencephalitis and an abdominal disease known as angiostrongiliasis. First detected in Cuba in 2014, ...