You can't say that. To do so is woefully ignorant. There are no OS X viruses that you know of. Since OS X is software, I assure you, a virus can be created for it, and since people are people, I assure you, one has been made for it. In any case - trojan, virus, malware, are all exceedingly bad and any of their presence on any system is, well, bad.
I can say aren't because there aren't. I'm not so naïve as to think that there will never be, certainly not! When one is created, almost all of us will get it because it will be without precedent. But there aren't any now, is all.