Your questions are not trivial if you don’t understand something and need answers. So, don’t worry about asking them, people here are happy to help, if they can.
As 1Q7FE6zp said, it is primarily about privacy as opposed to security. That is to say, the act of downloading via a bittorrent client in Linux is not a security issue in itself. The file you may have downloaded, however, could have security issues inherent in it. But, this would be an issue with the file, not bit-torrenting. In other words, it is the same issue as would be the case with any other file, however you obtained it.
The issue of viruses you mentioned is very low risk as compared to Windows. Viruses do exist, however. In which case, you could always belt and brace your system with a Linux anti-virus application. The most popular is ClamAV. See below for how to install it:
The main issue, though, is one of privacy. Your ISP can usually see what you are downloading as can, I presume, the seeds who are providing the file. In the event of downloading pirate media this can have legal implications. Or, in the case of sensitive political files, as with the kind of material handled by Wikileaks for example. In such circumstances, it pays to use a VPN which both renders you anonymous as well as encrypting the data on its journey to and from your PC.
All of the above, applies to any OS.