Verb (5-dan, る, intransitive)
1. to be popular; to come into fashion
これ、流行ってるのかな。I wonder if this is popular.
2. to be prevalent; to spread widely (e.g. disease); to be endemic
なんか風邪がいきなり流行りだしてるな。It seems like colds have suddenly started to spread.
3. to flourish; to thrive