Security Scan on macOS High Sierra

There is no security scan performed on your Mac by design. The operating system has built-in security functionality that safe-guards your Mac without input from you. For the same reason, there is no point installing so-called, anti-virus software, as there are no viruses on the Mac. The A/V software (pick a brand) has a long history of application interference, false positives, and entanglement with the built-in security functionality — causing misadventure of the operating system.

Depending on your browsing and download habits, you may need an interactive, free Malwarebytes scan for adware/malware that it will detect and remove said junk. If you do not download software from anywhere but the original vendor site, or through the Mac App Store, then there may be little need even for Malwarebytes.

Trusted software downloaded from a vendor site still needs to be verified by the operating system in a simple, one-time process:

1. Once the software is installed into the /Applications folder, you right-click on the application, and choose Open.
   1. A brief verification progress bar will appear, and then a second dialog will appear informing you that you have downloaded an application from the Internet. Click Open on that dialog too.
   2. The application will open, and you can use it, or quit it, and launch normally afterward.

An addendum to what VikingOSX writes: Effective defenses against malware and other threats - Apple Community