既是可数名词又是不可数名词，主要看具体表达的含义。Infection用来表达传染的时候，是不可数名词；infection用来表达传染病的时候，是可数名词。因此，infection是可数名词，也是不可数名词。
Infection的例句
Ear infections are common in pre-school children
耳部感染在学前儿童中很常见。
Exactly which bacteria cause the infection is still unknown.
到底是什么细菌引起的感染还不清楚。
There is lowered pain tolerance, lowered resistance to infection.
对疼痛的忍耐力降低了，对感染的抵抗力也减弱了。
Our doctor diagnosed a throat infection and prescribed antibiotic and junior aspirin
我们的医生诊断是喉部感染，并且开了一些抗生素和小剂量阿司匹林。
The main emphasis of our research has shifted to the prevention of infection and septicemia.
我们研究的重点转到预防感染和败血病方面来。