I would hope – with the many security issues surrounding them – that you do not use BitTorrent clients. Although a really neat idea, it is in the nature of BitTorrents that they are usually among the first, and sometimes the only, sources of malware.
This has been underlined once more: the Transmission BitTorrent client version 2.90 has been infected with ransomware, which will encrypt drives and demand payment to decrypt them.
Full technical details have been provided by Palo Alto Networks. This provides detailed recommendations for detecting the ransomware, if you are concerned that you may be affected. It is worth noting that the infected version of Transmission was provided from the official website, using HTTPS, proving another important security point.
If you are for any reason using Transmission, give version 2.90 a very wide berth and skip straight to 2.91 or higher, as fast as you can.
Apple has revoked the abused security certificate and updated its XProtect signature to block the ransomware, although you should clearly not rely on those to protect you.