This is a comment on what you should-and should not-do to avoid malware that circulates on the Internet and gets onto a computer as an unintended consequence of the user's actions. The short answer to the latter question is 'no,' but that answer may give the wrong impression that there is no threat to defend against. Mac users often ask what they should do to protect themselves from malicious software ('malware,' or loosely speaking, 'viruses') and in particular, whether they should use 'anti-virus' (AV) or 'anti-malware' software. In short, just use your new iMac and enjoy it. I converted from MS Windows 10+ years ago and have never had any malware issues and have gone through countless OS X updates and upgrades which all have gone perfectly. OS X runs reliably for years if left alone. If you get an e-mail or phone call advising you MUST download something, ignore it.In other words if it smells fishy, it likely is!

Don't download from a website you don't know or trust.Follow the instructions in Phony 'tech support' / 'ransomware' popups and web pages.Never download anything from a torrent site.However there is a very small amount of other malware out there however these are extremely easy to avoid by using a little common sense. OS X is extremely secure and there are no viruses for OS X. In a word no, you do not need any antivirus, cleaning or ANY, repeat ANY third party maintenance utilities. Do I also need to install AV software on the new iMac? and I had to have a good anti-virus software running all the time. My Windows PC was always vulnerable to malware, viruses, etc. I am a convert from the MS Windows world and this is my first Mac.

I just bought a new iMac running OS X 10.11.5 (El Capitan).

Malwarebytes el capitan