Antivirus software on MAC

I agree Malwarebytes is pretty good, and their precursor software for the Mac for older OS versions is good too.

Are you running Mac OS X 10.7?

Do not run any system optimizers. Do not use any illicit file sharing sites. Those are the places where the most virus related software happen. Do not download attachments from sources you don't know about. Make sure whomever sends you an attachment checks for viruses on the attachments before sending.

It's generally not useful; instead, avoid downloading software from sources other than the Mac App Store or the developer websites. If you need a product, click here.

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If you install an antivirus app it will definitely seriously slow down your computer and often causes other issues. It is absolutely not necessary. This forum is full of horror stories of people having installed an antivirus app.