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A virus message when opening an MP4 in PowerPoint on a Mac is most likely a security warning from PowerPoint itself that the video is external and could be harmful, not a real virus. To address this, embed the video directly into the presentation instead of linking to it, or adjust PowerPoint's security settings in the Trust Center to allow external content.
Embed the video: Instead of linking to an external MP4 file, use the Insert tab in PowerPoint, then select Video, and then This Device to embed the MP4 directly into your slide.
Adjust PowerPoint Trust Center Settings: Go to File > Options > Trust Center > Trust Center Settings.
Select External Content and then check the box for "Enable all data connections (not recommended)". Click OK to save your changes.