Some Antivirus have a conflict with the way some Fusion 2.5 extensions work and mark programs that use them as a virus.
An antivirus tries to guess a program is a virus and whenever they're not sure they may mark it as one. Avast in particular is a prime offender with false positives, but others as well do so.
If you google around you can see that a lot of Fusion made games, and sometimes Fusion itself, are treated as a virus even if they're not.
Unfortunately I have no way to stop AVs from marking the game as one. You have to whitelist it to play as normal.