Does my iMac require that I purchase an antivirus software?

I second the comment above. Macs have their own built in antivirus. Second, the Apple operating system is write protected so no it cannot be overwritten or changed, therefore it is immune to any virus. Third, an antivirus app will seriously slow down your computer and will often cause other operational problems.

No; instead, avoid downloading software from sources other than the Mac App Store or the developer websites.

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