It is far from true that computers running Linux or Mac OS cannot get viruses. I have personally witnessed viruses infecting macs many times. In fact, I know many people who don't have any antivirus programs running on their Macs because they believe the myth so strongly. The only solution I could find for their virus woes was to reinstall their OS and then try to convince them to get an antivirus program.
I find the biggest sources of viruses these days to be instant message programs like AOL, Yahoo, MSN etc. Then, of course there are still the sketchy websites that let you download naughty videos, free software that isn't supposed to be free, watch cable tv channels online for free, too good to be true spam messages in email,etc. These often cause viruses regardless of the OS.
The only reason why computers running the various Windows OS's seem to get all the viruses is because there are simply way more people using Windows than any other OS. Therefore, to get the biggest bang for their buck and the biggest terror result for their efforts (I name virus programmers as among our terrorist enemies, more needs to be done to catch these guys!), virus programmers write most of their viruses for Windows. Think about it. If you were a bomber, would you blow up an outhouse in a remote area of a national park, or would you blow up a convention hall packed with conference attendees? Which would scare and hurt more people? The same logic holds true with viruses.
As more and more people start getting ipads, iphones, macbooks, mac PCs etc. we will see more and more viruses written for Mac. Mac use is on the rise, so it's only a matter of time.
I don't know if Linux use will ever take off. It's too complicated and frustrating for the average person just to get Linux to install and then to do basic functions that most people don't bother with it. If they ever manage to make LInux as user friendly for the average joe as say Windows or Mac and if video game and productivity software companies start making Linux compatible programs (sure you can force Linux to run Windows based programs, but it's not easy), then it might become more widely used.
In the mean time, regardless of what OS you use, get an antivirus program. Sure they aren't 100% effective, but that is because the evil virus writers relentlessly keep coming up with new viruses. It's like trying to come up with a vaccine to fight off a disease that is constantly mutating and becoming immune to whatever vaccine you develop faster than you can get the FDA to approve the vaccine for use.
The best defense against viruses is to try to only download stuff from trusted sources and avoid the scams and sites with illegal content. Abstinence is more effective than using a condom.
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