It's not so much that game trainers can "cause" viruses -- although anything you download from the Internet could be compromised or have a payload attached to it, so be careful.
Game trainers also can trigger detection by security software. For example, ours has real-time behavior protection (Behavior Shield) that may report apps such as game trainers as malicious, due to the way they behave on your machine.
You can mark these apps as safe with exclusions, but only do so if you are 100% certain they are safe to use.
More info on Behavior Shield if you're curious: https://blog.avast.com/behavior-shield-our-newest-behavioral-analysis-technology