Uploading and downloading of copyrighted material, usually music, films or 3rd party software programs like MS office etc. is in most countries illegal. The action of downloading a torrent does exercise your hard disk in that it is constantly reading and writing from/to your disk. This will inevitably have an impact but then again thats what the hard disk is meant to do. If you do this on a laptop then as stated above make sure that your machine is well ventilated. Downloading a torrent with a virus/trojan/malware will only affect your machine if you activate it and it was designed to infect your particular OS. If it's written for a Windows machine probably (99.999% are and thats just my straw figure btw) you are ok, if written for a mac then you are out of luck.
If you have a NAS such as a Synology branded one then you can let that do your torrenting for you. The Synology NAS by default will stop seeding/sharing any torrents as soon as your download completes, which will reduce your exposure considerably to authorities who join swarms to track infringers.
Essentially legitimate torrenting is just another way of getting data onto your machine as opposed to via USB drive, CD/DVD etc. and nothing more. If the data is benign then it won't harm your machine and if it's malicious and written specifically for a Mac then it might. Just take the same precautions you would take when putting any data onto your machine and aside from the increased wear and tear you will be fine.
For further info regarding how to download torrents, how to spot fakes prior to downloading, how to spot a quality torrent from a low quality one, you are probably better off joining a torrent forum.