Do I need anti virus ?

No. There are no extant viruses in the wild. But you will find some malware.

Helpful Links Regarding Malware Problems

If you are having an immediate problem with ads popping up see The Safe Mac » Adware Removal Guide and AdwareMedic.

Open Safari, select Preferences from the Safari menu. Click on Extensions icon in the toolbar. Disable all Extensions. If this stops your problem, then re-enable them one by one until the problem returns. Now remove that extension as it is causing the problem.

The following comes from user stevejobsfan0123. I have made minor changes to adapt to this presentation.

Fix Some Browser Pop-ups That Take Over Safari.

Common pop-ups include a message saying the government has seized your computer and you must pay to have it released (often called "Moneypak"), or a phony message saying that your computer has been infected, and you need to call a tech support number (sometimes claiming to be Apple) to get it resolved. First, understand that these pop-ups are not caused by a virus and your computer has not been affected. This "hijack" is limited to your web browser. Also understand that these messages are scams, so do not pay any money, call the listed number, or provide any personal information. This article will outline the solution to dismiss the pop-up.

Quit Safari

Usually, these pop-ups will not go away by either clicking "OK" or "Cancel." Furthermore, several menus in the menu bar may become disabled and show in gray, including the option to quit Safari. You will likely have to force quit Safari. To do this, press Command + option + esc, select Safari, and press Force Quit.

Relaunch Safari

If you relaunch Safari, the page will reopen. To prevent this from happening, hold down the 'Shift' key while opening Safari. This will prevent windows from the last time Safari was running from reopening.

This will not work in all cases. The shift key must be held at the right time, and in some cases, even if done correctly, the window reappears. In these circumstances, after force quitting Safari, turn off Wi-Fi or disconnect Ethernet, depending on how you connect to the Internet. Then relaunch Safari normally. It will try to reload the malicious webpage, but without a connection, it won't be able to. Navigate away from that page by entering a different URL, i.e. www.apple.com, and trying to load it. Now you can reconnect to the Internet, and the page you entered will appear rather than the malicious one.

An excellent link to read is Tom Reed's Mac Malware Guide.

Also, visit The XLab FAQs and read Detecting and avoiding malware and spyware.

See these Apple articles:

Mac OS X Snow Leopard and malware detection

OS X Lion- Protect your Mac from malware

OS X Mountain Lion- Protect your Mac from malware

OS X Mavericks- Protect your Mac from malware

About file quarantine in OS X

If you require anti-virus protection Thomas Reed recommends using ClamXAV. (Thank you to Thomas Reed for this recommendation.)

From user Joe Bailey comes this equally useful advice:

The facts are:

1. There is no anti-malware software that can detect 100% of the malware out there.

2. There is no anti-malware that can detect everything targeting the Mac.

3. The very best way to prevent the most attacks is for you as the user to be aware that

the most successful malware attacks rely on very sophisticated social engineering

techniques preying on human avarice, ****, and fear.

4. Internet popups saying the FBI, NSA, Microsoft, your ISP has detected malware on

your computer is intended to entice you to install their malware thinking it is a

protection against malware.

5. Some of the anti-malware products on the market are worse than the malware

from which they purport to protect you.

6. Be cautious where you go on the internet.

7. Only download anything from sites you know are safe.

8. Avoid links you receive in email, always be suspicious even if you get something

you think is from a friend, but you were not expecting.

9. If there is any question in your mind, then assume it is malware.

flávio danny wrote:

Do I need anti virus for my Mac Book Air?

Mac OS X has built-in security. For more information, check out this page. Sophos (Free) and Bitdefender Antivirus (Paid) are two good options for additional protection.

No. AV software on Mac will only cause far more problems then is can solve.

Do I need anti virus for my Mac Book Air?

No; nothing that is not already incorporated in OS X. Third party (non-Apple) "anti-virus" products convey no additional benefit. Most of them cause problems. Some of them are malicious.

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.
  • Popups themselves cannot infect your Mac, but many contain resource-hungry code that will slow down Internet browsing.
  • If you ever receive a popup window indicating that your Mac is infected 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 or other copyrighted content that is not normally "free".
  • The more insistent it is that you upgrade or install something, the more likely it is to be a scam. Close the window or tab and forget it.
• 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.