ひよりみかんせん

日和見感染

Opportunistic infection

健康な人には感染症(infectious disease)を起こさない菌が原因となって、免疫力(immunity / immune system resistance to illness)が低下した人に発症すること。

This refers to an infection that is caused by bacteria (germs/viruses) that would not cause an infection in a healthy person, but occurs in people who have lowered immunity.