Why do I keep getting a message about viruses on my iPhone?

They're phishing scams, designed to encourage you to pay for software/services you don't need for issues you don't have. If you know which sites are sending these scam messages to you, STOP visiting those sites. They are PAID by the scammers to send you the messages.

Bottom line is there are no known viruses for non-jailbroken iPhones. Read this --> Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

Those are fake virus messages, and they are coming from the web pages you are visiting and not your iPhone. Your iPhone doesn't send those kinds of messages out, and even so your iPhone isn't susceptible to viruses as long as they aren't jailbroken. Simply close out of those web pages and do not interact with them in any way. They are likely trying to get you to purchase unnecessary software, give away your personal information, or your credit card information.

Read these:

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

How to Spot Real Apple Communication and Watch Out for Scams - Apple Community

They are fake.

Those "certain sites" are accepting payment from scammers to "advertise" unneeded security products through fake "You're infected" messages. Avoid those sites and don't click the messages.

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

Block pop-up ads and windows in Safari - Apple Support

No one—not even the Apple demigods on Mount Cupertino—can "see" what is one your device without your granting permission for one-time remote access.

creourakd51 wrote:

On certain sites on the internet I will get a message stating that my I phone has several viruses

[Re-Titled by Moderator]

It’s all about crooks wanting your money. Ignore those messages.

The messages are from scammers. Fall for them, get ripped off, and your name and financial information may get sold to other criminals who would like to rip you off later …

U.S. Federal Trade Commission – How To Spot, Avoid, and Report Tech Support Scams

"Tech support scammers may try to trick you with a pop-up window that appears on your computer screen. It might look like an error message from your operating system or antivirus software, and it might use logos from trusted companies or websites. The message in the window warns you about a security issue on your computer and tells you to call a phone number to get help.

…

If you get this kind of pop-up window on your computer, don’t call the number. Real security warnings and messages will never ask you to call a phone number."