Trojan.Agent refers to a generic Trojan malware detection. It is used for detections that are either associated with no specific malware families or not enough information is available to pinpoint the malware family. Often, you see it paired with other detections as parts of an infection that goes beyond a single family.
Since this is a generic detection, this threat is not confined to any particular infection method. It is advised to check other detected items for more detail on how the threat got on your system. Otherwise, following basic security practices will reduce your exposure to this threat.
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