Viruses attached with my iPhone calender

Open your calendar app and tap Calendar at the bottom, if there is a calendar you do NOT know, tap it and delete it. You can also go to Settings -> Calendar -> Accounts. If there is an account you do not know, delete it immediately.

On some websites there are pop ups where there are invitations to participate in a kind of "SPAM calendar" you have clicked add without thinking about it. I have tried it myself and you just delete it again.

Open Calendar App from the Home screen.

Next, tap the Calendars button at the bottom.

You should now notice a section called On My iPhone with several suspicious calendars listed below it. ...

Tap on the i button next to the suspicious calendar.

Now, tap on Delete Calendar.