virus or something

To confirm, you mean 13.3 by “the latest”?

Which iPhone?

Scroll all the way thru the entry, as sometimes the settings are past the end of the current display.

Reboot: Restart your iPhone - Apple Support

I’m assuming that’s an entry you’ve added.

If it’s not, you may be looking at an invite. Usually embedded in email.

Those get accepted or rejected. Not deleted.

And accepting or rejecting sends a notification to the spammer.

Which is bad.

Send and receive invitations in Calendar on iPhone - Apple Support

I disable automatically processing those, as they tend to be spam.

Log into iCloud.com, select Calendar, select settings (the gear), Preferences, Advanced, use email and not in-app.

Had similar problem.

resolved by going into privacy settings, than calendar.

unsubscribe by clicking on calendar (usually the malware). This will resolve your issue.

As a future precaution I updated calendar settings as pointed out on thread below to not automatically add but send as email.

Which version of iOS? 13.3?

Tried rebooting the iPhone?

Malware on an iPhone is pretty rare, assuming it’s not been jailbroken.