"Your iPhone is severely damaged by (13) viruses"

Such messages are scams.

Close the offending page, and avoid sites that are involved in such bogus advertising tactics.

Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

Effective defenses against malware and ot… - Apple Community

There are HTML coding techniques that make a window open behind existing windows without active cooperation and regardless of what pop-up blockers may have been installed or activated, often through use of Javascript.

I've been getting this notice a lot more often lately which is why I came here to check if it could be real. It's a pop up from different pages I open from news stories and often when I click a link from Facebook

their goal is to get you to willingly provide them with your appleID and password and probably your creditcard info

you can read more about this type of scams here https://en.wikipedia.org/wiki/Phishing

It's usually a pop-up ad of some type masquerading as an error page.

Just close the page and/or Safari, and don't click on links or enter any information.

If it makes you feel better, but these sites work on information you enter online, they don't affect your phone itself.