他们的第四个孩子在婴儿时期就夭折了。

Their fourth child died in ___infancy.

在古罗马时期，许多熬过了婴儿期的人也活不过四十岁。

In Roman times, many who survived ___infancy did not live past forty.

这种疾病通常在婴儿期感染，但可能直到成年才会出现症状。

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