Many of the top private trackers rely on reputation. Which is why some of them are notoriously hard to get into. They usually also have active mods, admins and forums. They also generally have posting rules for torrents. Anyone wanting to spread malicious torrents would likely get their account banned in short order once it gets reported. not to mention quite a number of them have their own internal release groups that can be trusted.

While it's not impossible, it's far less likely to be an issue on any well run private tracker.