Q: What is a good anti virus system for yosemite?

The defenses built into Mac OS X and common sense. Unless you're running Windows on the computer or frequently exchange files with people using Windows, you don't need any other anti-malware software.

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As our Wisest Member advised - but did not stress - common sense (or lack thereof)  and Windows OS - are the biggest weaknesses in your computer's security.

There are many forms of ‘Malware’ that can affect a computer system, of which ‘a virus’ is but one type, ‘trojans’ another. Using the strict definition of a computer virus, no viruses that can attack OS X have so far been detected 'in the wild', i.e. in anything other than laboratory conditions. The same is not true of other forms of malware, such as Trojans. Whilst it is a fairly safe bet that your Mac will NOT be infected by a virus, it may have other security-related problem, but more likely a technical problem unrelated to any malware threat.

You may find this User Tip on Viruses, Trojan Detection and Removal, as well as general Internet Security and Privacy, useful: The User Tip seeks to offer guidance on the main security threats and how to avoid them.

https://discussions.apple.com/docs/DOC-2435

More useful information can also be found here:

http://www.thesafemac.com/mmg/

https://www.malwarebytes.org/

These articles from MacWorld are also useful:

http://www.macworld.co.uk/feature/mac-software/mac-antivirus-internet-security-s oftware-malware-review-3523842/

http://www.macworld.co.uk/how-to/mac-software/are-macs-safe-viruses-hackers-3454 926/

Install no such thing. Read below.

There will always be threats to your information security associated with using any Internet - connected communications tool:

1. You can mitigate those threats by following commonsense practices
2. Delegating that responsibility to software is an ineffective defense
3. Assuming that any product will protect you from those threats is a hazardous attitude that is likely to result in neglecting point #1 above.

OS X already includes everything it needs to protect itself from viruses and malware. Keep it that way with software updates from Apple.

A much better question is "how should I protect my Mac":

• Never install any product that claims to "clean up", "speed up", "optimize", "boost" or "accelerate" your Mac; to "wash" it, "tune" it, or to make it "shiny". Those claims are absurd.
  • Such products are very aggressively marketed. They are all scams.
    
• Never install pirated or "cracked" software, software obtained from dubious websites, or other questionable sources.
  • Illegally obtained software is almost certain to contain malware.
  • "Questionable sources" include but are not limited to spontaneously appearing web pages or popups, download hosting sites such as C net dot com, Softonic dot com, Soft pedia dot com, Download dot com, Mac Update dot com, or any other site whose revenue is primarily derived from junk product advertisements.
  • If you need to install software that isn't available from the Mac App Store, obtain it only from legitimate sources authorized by the software's developer.
• Don’t supply your password in response to a popup window requesting it, unless you know what it is and the reason your credentials are required.
• Don’t open email attachments from email addresses that you do not recognize, or click links contained in an email:
  • Most of these are scams that direct you to fraudulent sites that attempt to convince you to disclose personal information.
  • Such "phishing" attempts are the 21st century equivalent of a social exploit that has existed since the dawn of civilization. Don’t fall for it.
  • Apple will never ask you to reveal personal information in an email. If you receive an unexpected email from Apple saying your account will be closed unless you take immediate action, just ignore it. If your iCloud, iTunes, or App Store account becomes disabled for valid reasons, you will know when you try to buy something or log in to this support site, and are unable to.
• Don’t install browser extensions unless you understand their purpose:
  • Go to the Safari menu > Preferences > Extensions. If you see any extensions that you do not recognize or understand, simply click the Uninstall button and they will be gone.
• Don’t install Java unless you are certain that you need it:
  • Java, a non-Apple product, is a potential vector for malware. If you are required to use Java, be mindful of that possibility.
  • Java can be disabled in System Preferences.
  • Despite its name JavaScript is unrelated to Java. No malware can infect your Mac through JavaScript. It’s OK to leave it enabled.
  • The same precaution applies to Adobe Flash Player. Newly discovered Flash vulnerabilities appear almost weekly.
• Beware spontaneous popups: Safari menu > Preferences > Security > check "Block popup windows".
  • Popup windows are useful and required for some websites, but unsolicited popups are commonly used to deceive people into installing unwanted software they would never intentionally install.
  • The mere appearance of a popup itself does not infect your Mac with anything malicious, but many contain resource-hungry code that will slow down Internet browsing.
  • If you ever receive a popup window indicating that your Mac is infested with some ick or that you won some prize, it is 100% fraudulent. Ignore it.
  • The same goes for a spontaneously appearing dialog insisting that you upgrade your video player right this instant. Such popups are frequently associated with sites that promise to deliver "free" movies, music, or other copyrighted content that is not normally "free".
  • If you find Safari has locked up, leaving you unable to dismiss the page, read Phony "tech support" / "ransomware" popups and web pages for the solution.
• Ignore hyperventilating popular media outlets that thrive by promoting fear and discord with entertainment products arrogantly presented as "news". Learn what real threats actually exist and how to arm yourself against them:
  • The most serious threat to your data security is phishing. Most of these attempts are pathetic and are easily recognized, but that hasn't stopped prominent public figures from recently succumbing to this age-old scam.
  • OS X viruses do not exist, but intentionally malicious or poorly written code, created by either nefarious or inept individuals, is nothing new.
  • Never install something without first knowing what it is, what it does, how it works, and how to get rid of it when you don’t want it any more.
  • If you elect to use "anti-virus" software, familiarize yourself with its limitations and potential to cause adverse effects, and apply the principle immediately preceding this one.
  • Most such utilities will only slow down and destabilize your Mac while they look for viruses that do not exist, conveying no benefit whatsoever - other than to make you "feel good" about security, when you should actually be exercising sound judgment, derived from accurate knowledge, based on verifiable facts.
• Do install updates from Apple as they become available. No one knows more about Macs and how to protect them than the company that builds them.

Summary: Use common sense and caution when you use your Mac, just like you would in any social context. There is no product, utility, or magic talisman that can protect you from all the evils of mankind.