I'm getting messages on my iPad that my device is currently infected

Apple's calendar does that. It wasn't the whole calendar that was giving you the message it was one subscription calendar that you picked up from a web site. Apple has it's own subscription birthdays calendar and you don't need a special one to enter events. But if you want to make things easier look into subscribing to Fantastical and using their app.

Terri-bill wrote:

I did that and got the same message your iPhone is infected

Subscribed calendars are used by various organizations such as schools and sports clubs and volunteer organizations to share information on scheduling and events, whether for events or for sporting games, or for coordinating volunteers, or who is feeding the cats at the local cat shelter.

What happened here? You were offered a calendar subscription from a website or mail message or such, and you accepted the subscribed calendar, and the subscribed calendar is filled with spam.

To remove the spam-filled subscribed calendar, access the list of subscribed calendars and unsubscribe from the subscribed calendar filled with spam.

Once the spam-filled subscribed calendar is no longer subscribed, the spam from the subscribed calendar will no longer trouble you.

There is nothing wrong here, nothing to fix, no hacks, no virus, nothing other than a calendar filled with spam. Unsubscribe.

Access the list of subscribed calendars, and remove the unrecognized subscribed calendars. Some of the miscreants are naming their calendars to try to confuse the removal of the subscribed calendar, so look at the color of the subscribed calendar and the color of the spam messages. In the photo above, the subscribed calendar is deliberately named “events”, and the subscribes calendar and its events are all colored yellow.

Delete spam calendars and events on iPhone - Apple Support

It is a scam message. Is this in a Message, eMail, pop up or in your Calendar?

I have no idea of what calendar you are referring to.