expression, godan verb (-ru), intransitive verb
get worse; deteriorate; become worse
General worsening of a situation, condition, or quality. Used for weather, health, relationships, performance, etc.
天気が悪くなってきた。
The weather is getting worse.
彼の体調が急に悪くなっ
His physical condition suddenly deteriorated.
expression, godan verb (-ru), intransitive verb
Specifically for food or perishable items becoming inedible or rotten.
冷蔵庫に入れないと、すぐに悪くなるよ。
If you don't put it in the fridge, it'll go bad quickly.