Despite what you may have heard, Macs are not immune to being infected by malicious software (malware). While malware infections are less common on Macs than on PCs, Macs are still vulnerable to adware, spyware, viruses, worms, trojans, and other unwanted programs. This guide will give you some recommendations for what actions to take if you suspect you have malware on your system.

Are you infected?

It's not always easy to tell if you have an unwanted piece of software on your system, but there are some signs you can look for:

  • Your browser is opening pop-up advertisements on websites that don't normally have pop-up ads
  • Websites you normally visit have randomly linked words in the text of the article. When you hover over these links, advertisement windows appear
  • Your browser is showing inappropriate advertisements on sites that don't normally display ads of this type.
  • You notice a sudden, unexpected decline in computer performance. Applications and websites respond slower and you computer takes much longer to start up or shut down.
  • When you perform internet searches, the default search engine isn't Google, DuckDuckGo or Bing.
  • Here's a more detailed run-down of other symptoms of Mac malware

Disable Browser Add-Ons

A lot of adware is distributed as malicious browser plug-ins. Go through your preferred browser's list of add-ons and plugins and try disabling or removing anything that looks fishy. If you're not sure, you can also just try disabling ALL of the add-ons.

Try disabling add-ons for your preferred browser and see if it starts behaving normally again.

Run A Malware Removal Tool

There are several vendors who produce antimalware software for the Mac. Malwarebytes offers a good product that also happens to be free for personal use. Download the installer, drag the "Malwarebytes" icon into your Applications folder and double-click it to run a scan. Follow the instructions to remove any identified malware.

Do a Manual Search

This guide from Apple lists many well-known malware programs that are known to affect Apple products. If you're still having issues with your machine, read through this list and see if any of the listed files are present on your machine: Apple Adware Removal Guide.  Here's one more guide to identifying and removing adware by hand: SafeMac Adware Removal Guide

Restore from a Time Machine Backup

If you're having persistent problems with installed malware, you may need to completely restore your computer from a Time Machine Backup. If you use Time Machine (and you should, here's how), you can reset your computer to a time before you were infected by malware. Follow these instructions to restore from a Time Machine backup. When asked what backup you'd like to use, choose the most recent date listed that falls before your computer started having issues. Keep in mind: any documents, applications, or downloads added to your Mac since this date will be lost!

Reinstall Your Operating System

As a last resort, you can also try the nuclear option: a complete re-installation of your operating system. Keep in mind that a complete reinstall of Mac OS X will delete everything on your computer. Every document, photo, song, movie, and application that isn't backed up will be lost forever. As such, only consider this option if you've tried everything else on this list and you're still having problems. When you're finished, your Mac will behave as if you just picked it up from the store. Here are two articles describing how to do this, from Gizmodo and from Apple.  Keep in mind that you'll need an internet connection and several hours to complete this process.