why in apple there is no antivirus and why it is not shareable with other OS.
Well, honestly, I can see why nobody answered this: it feels like a troll, and your bio line is basically an advertisement.
But I’ll take a crack at it.
It is true that Mac OS X doesn’t have antivirus utilities built into the operating system. The same is true of Linux and FreeBSD, after all, and it shouldn’t be surprising, since Mac OS X is built on Unix underpinnings. The Unix security model has some advantages over the Windows security model, mostly because it doesn’t make so many compromises for convenience (and I still maintain that Window’s SYSTEM account is a baaad idea).
That said, like Windows, there are antivirus utilities out there, either for free or for purchase, for Mac OS X.
As to the second part of your question, it doesn’t really make any sense. Files are shareable, of course; any Mac OS X system can share files either to other Macs using AFP, or to Windows boxes using SMB. If you prefer to share via webserver, you can have that, too—it comes with the operating system. Since it’s running a variant of FreeBSD under the hood, it also shares not just a Unix heritage, but also thousands of Unix applications which can be easily managed with (say) MacPorts.
Whatever the case, perhaps you want to learn more about Mac OS X before asking your next question. While I am more fond of open-source offerings, I have to admit that the Mac operating system is Unixy enough for me to get actual work done without jumping through too many hoops, while having a more robust, solid, well-thought-out interface than any Unix GUI. While I wish I could give my 80-year-old father a FreeBSD/KDE system and have him use it, the fact remains that it’s just not as intuitive or consistent as the Mac GUI, and he can get around pretty handily on that by himself.
ClamAV is free, open source, and quite good. There are other options out there as well, but commercial ones that will cost you some $$.