Do I need to purchase an additional anti virus program for my MAC computer

No, the macOS has Built-in Security and all you need to do is keep it up to date.

see > https://www.apple.com/macos/security/

Absolutely not! Adding third party anti virus, cleaning or any other third party maintenance apps are at best a waste of time and money and at worst will cause your Mac to be slow, act odd and be unstable. Please simply keep your iMac up to date and otherwise just enjoy it.

No.

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