流感病毒在拥挤的地方传播得很快。

A flu ___virus can spread quickly in crowded places.

我不确定她到底怎么了——我想是某种病毒。

I don't know exactly what's wrong with her - I think it's some kind of ___virus.

医生说是病毒引起了这种疾病。

The doctor said it was a ___virus that caused the illness.