Trend, Fad, Craze, Prevalence
noun, verbal noun, の adjective
Where the stream is going is where everyone's going. That's what tends to be the trend of things. Imagine a trend going on in your society right now. Imagine it going down a stream with everyone following it. This word is also used for less desirable trends, like the prevalence of infectious diseases.
りゅうこう, はやり
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
ショッピングモールでは流行の服を売っています。
Shopping malls sell trendy clothes.
マレットは流行のヘアスタイルの一つです。
A mullet is one of the trendy hairstyles.
この冬、お年寄りの間にインフルエンザが流行しているらしい。
Apparently influenza is prevalent among the elderly this year.