virus

It's a scam. There are no known viruses for iPhone.

If the message is coming form a website in a web browser, ignore it.

If it's coming from a calendar event, then it means you subscribed to a scam ware calendar and its pushing these notifications. If this is the case we can tell you how to remove it.

Otherwise, explain exactly where you are seeing these messages that say you have a virus.

Scam message. Ignore it. A non-jailbroken phone cannot be hacked or get a virus.