I have a message “ your phone is infected

If you are seeing this in your calendar, see below. Otherwise, post back with relevant details.

If running iOS 13 or earlier versions, check: Settings - Passwords & Accounts - Accounts - Any rogue entries here? If so, delete the rogue account. 

If running iOS 14, check: Settings - Calendar - Accounts - Any rogue entries here? If so, delete the rogue account. 

Check this first:

Remove spam from Calendar on your iPhone - Apple Support

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support