How can you get rid of viruses on your iPhone?

There are no known viruses in the wild that can infect an iPhone that has not been jailbroken.

Because you saw one of thousands of such bogus messages that people see everyday.

Where did you actually see the message?

Website? If so, avoid such websites.

If you saw this “warning” while visiting a website, then it is usually the result of an advertisement. Advertisement systems can use JavaScript, and malicious advertisement designers can take advantage of JavaScript to produce false messages.. such as your virus warning. Ignore it.

If you saw the message in a Calendar event, then see here to remove them: link-> Delete spam calendars and events on iPhone - Apple Support