Bitlet and Torrentrelay are just host sites for the torrent file; they don't store or upload the data that the torrent actually "refers" to.
There is some security to torrents. Once a torrent is created it can't be changed. Altering file sizes or file names corrupts the torrent and nobody can download or upload a corrupt torrent. Comparing a torrent's hash with a known original, you know the data you're getting are the same data that were the original torrent.
BUT, if the original data the torrent refers to is corrupted, like virus infected or an unplayable file, you'll get the same data including the viruses and corruptions.
Just virus scan the data after you download all of it but before you actually use it. If the scanner is, you know... reputable and current... you'll have some assurance it's not some big virus carrier.