In addition the answers above, I would like to suggest using a Virtual Machine.
Examples of such software that runs on Mac include Oracle's VirtualBox, Parallels Desktop and VMWare Fusion - pick any of them (or any of alternative ones) that suits your needs (it's probably worth checking their security as well).
The advantage of using a Virtual Machine is that if by some occurence some malware ends being downloaded and/or installed on "virtual" OS, it affects only such system, leaving the base system safe. At least in theory, since there have been some breaches found in Virtualization software, however such attacks aren't that easy to carry out. So while it doesn't offer 100% safety it offers safety significant enough to consider it. I believe that the malware you mentioned can't move between Virtual Machines/to base OS (at least a quick Google search seems to suggest that).
The way I believe would suit your needs would be to install some Virtual Machine Software, run a virtual OS using it, and use such OS for the purposes of downloading files/programs and using them. For example, you would download the Transmission BT client to your virtual OS and ideally run it from such environment. You could still use your Mac for regular uses, or - if safety is an extreme priority, you could refrain from downloading software to the base OS (OS X in this case) altogether. Like I said, this doesn't guarantee complete safety - but it does offer a certain degree worth considering.
Last note - while the technique above can provide advantage, it should not be treated as a substitute for protecting important data through backups (in example cold storage backups, having ones on multiple separate devices etc.).