The disease is often contracted in infancy but may not manifest symptoms until adulthood.

infancy 是名词 infant（婴儿）加上后缀 -cy 构成的名词形式，表示"婴儿期"或"幼年阶段"。infant 来自拉丁语 infans，意为"不会说话的"，指刚出生到一岁左右的幼儿。