Hello, fellow Mac user.
If you’re reading this, then you are looking for the definitive answer to whether or not you need a third party antivirus application installed on your Mac. The short and sweet answer is “yes.” If you want the long answer, it too is “yes,” but with complexities sprinkled in.
Let's start with the funny Mac vs. PC commercials that took over our televisions in 2006. Watching these commercials, you may have heard that Macs are impervious to viruses, which is simply not true. In the commercial that deals with viruses, the PC actor states that there were 114,000 known viruses for PCs, while the Mac (Justin Long) said, “PCs, but not Macs.” While this is true, it is slightly misleading because there are 30 pieces of malware  as of June 2015 for your beautiful Macintosh. As you can see the chasm is rather vast between these numbers, and can lead one to believe that antivirus on your Mac is a moot point, but don’t let the small number fool you. It is just as easy to download a malicious application to your Mac from a website, as it is on a Windows system. To be brief, computers, in general, are susceptible to malware because of you, the human element.
In fact, we humans help malware along because most of us have a tendency to just quickly click the “Yes” button when we are trying to install a program or make a popup go away. This trend is ideal for nefarious characters lurking on the net, but sorry for the rest of us because grandma's infected text document is now destroying our computers. That said, viruses are not the only pieces of bad code that you should concern yourself with. Now, before you throw on your deerstalker hat (that slightly unfashionable, double-billed cap that Sherlock wears) and start inspecting files on your Mac, you should know a few things about malware. It is crucial to be at least familiar with these terms: malware, viruses, Trojan horses, exploit, worm, spyware, spam, and Botnet. Since each definition would take up more space in this document than you most likely would care to read, I will leave it to the security gurus at Intego to give you the exact definitions: Intego link
If for some reason you don’t trust clicking the link above (smart to be cautious), please copy and paste the URL below:
After reading their definitions, it is easy to see that the term virus does not mean malware because malware is the term for all malicious codes. Keep this pearl of information in your pocket because it will help you decipher all the mixed information that you get through media outlets or social networking sites. Now that you are aware there are more than just viruses lurking on the internet, which can harm your Mac, let’s hit this point home by reading a few quotes from Titans in the computer security industry:
“I still prefer OS X, but due to ease-of-use and customization, not security reasons. In my mind, the OS question is quickly becoming moot, and will soon be replaced by the already-intense Web browser holy wars…” (Mike Bailey, Senior Researcher at Foreground Security)
“Mac malware is becoming more common, is in the wild, and is financially motivated...You can still get hit--but there are a lot less arrows being thrown at Mac users...I do tell my friends that they should run antivirus on their Macs, just like I do on the Macs my wife and I use at home.” (Graham Cluley, Senior Technology Consultant at Sophos) 
“The risk landscape for consumers (and enterprises) has changed over the last few years. Operating systems as such are no longer the primary target of consumer-targeted attacks; applications are.” (Frank Heidt, CEO of Leviathan Security) 
"I have a Mac. Having a Mac is more secure because not that many people have Macs. I think their market share is still less than 15 percent. For every point of market share, the risk goes up. Mostly I have a Mac because it is a better machine, not because it is more secure.” (Gary McGraw, Chief Technology Officer at Cigital) 
Despite this information, some individuals will forego third party antivirus because they think their Macs already have such features. It's true that since Leopard (OS X 10.5) Apple integrated a rudimentary antivirus called File Quarantine  into the operating system, but it's not what you would consider a modern antivirus application. If you have a modern Mac you most likely have Snow Leopard (10.6) - MacOS Sierra (10.12) installed on it. In these operating systems, Apple increased what File Quarantine can scan for , but it’s still not what you expect from a modern day antivirus application. In fact, Apple carefully states multiple times in its article HT201940 that only “known” malware is detected. That leaves you unprotected against unknown malware that Apple hasn’t seen or updated File Quarantine to detect. Usually, third-party antivirus applications have heuristic scanning or application behavior analysis capabilities, so as to try and make an educated guess about possible unknown malware sneaking around on your Mac.
To sum everything up, think of File Quarantine on your Mac as the standard locks on the doors in your home. While they keep people out to a certain level, they are not the best solution for security. Instead, you can increase the protection of your house by purchasing a home security system (glass break detector, motion sensor, door & contact sensors, cameras), which in this case, is a third-party antivirus application for your Mac.
Now that you have the knowledge about why you should protect your beloved Mac, please go and do so.
Side note: What does Academia say? Below, are what my universities state about secure computing on PC and Mac computers. You will notice that each one requires third party antivirus for any system wanting to connect to the campus network.
New York Institute of Technology:
 “Mac Myths and Facts.” Mac Myths and Facts. ESET. Web. 22 June 2015
 Mills, Elinor. "In Their Words: Experts Weigh in on Mac vs. PC Security - CNET." CNET. CNET, 1 Feb. 2010. Web. 22 June 2015.
 “Apple.” About the “Are You Sure You Want to Open It?” Alert (File Quarantine / Known Malware Detection) in OS X. Apple, 13 Feb. 2015. Web. 22 June 2015