I have a virus in my calendar

It's not a virus. iPhones cannot get viruses. Rather, you clicked on a link that was actually an invitation to a spam calendar.

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. 

Remove spam from Calendar on your iPhone

Are you seeing something as shown in the pic below? This is neither a hack nor a virus. It is spam and you can get rid of it. Read below...

Then probably you have accepted an offer or subscribed for a spam Calendar accidentally. Watch the GIF below to know how to unsubscribe from these alerts. Look for an unknown Calendar on the device under subscribed calendars.