The 27 viruses alert is not real and is a well known scam.
The 27 viruses alert is e pop up that is designed to scare you into clicking on links or downloading of dubioud software, and often with PUPs, "potentially unwanted programs" like MacKeeper.
Actual real viruses on your phones and computers do not trigger these pop-up, which are created by scam websites, and not your device's operating system.
The best response when you get these virus alerts and pop ups is to close your tab or browser and sometimes you might need to restart your computer.
It does not always mean you even have a virus on your computer or device when you get these pop ups from your browser although it's a good idea to scan your computer or device after clicking on a link and getting those pop ups to be sure there's no virus on your computer or device.
Websites though cannot scan your computer or device to genuinely find viruses and so these messages are browser based tricks.
The pop ups claim that sensitive data, like photos, banking information etc is compromised to create panic.
The push you to download specific apps, often with potentially unwanted programs that promise to fix the problem, but are often useless or harmful.
While common on Apple devices, iPhones and Macs, have similar scams which target Android and PC users as well.
If you do see the 27 viruses alert and other virus alerts or pop ups on your computer, phone or device don't click on anything and don't interact with the popups or it's links.
Force close the browser if the pop up won't go away and clear your browser's history and website data to remove any lingering cookies.
And avoid websites that use the aggressive ads in the future.