Think of the way a virus can spread - that is going viral. Something that spreads from person to person, by word of mouth or media account to media account, 'to spread like wildfire' is another synonym or metaphor of similar meaning

It is a new expression meaning something that becomes hugely popular on the internet, a short video or a picture that people share with each other very fast. It does come from the word "virus" but today this expression is only used in this specific way to talk about a popular internet trend.

1. of, relating to, or caused by a virus. Or 2. pertaining to or involving the spreading of information and opinions about a product or service from person to person, especially on the Internet or in emails. Some YouTube videos get really popular over night and go viral.

I would say a good alternative meaning would be "To become popular" Did this help?