The 14 Days With You demo is free! You don't need to pay for it.
If you're talking about the DLC, please keep your comments to the DLC page instead!! Posting them here can confuse new people who don't know anything about the DLC.
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Yes, that's normal for most indie developers. And you're welcome to run 14DWY through any virus-detection programs if you'd like. Though in the future, please make sure to download the game only from this Itch page; I'm not sure why you would've gotten any kind of virus unless it was downloaded from a shady scam/phishing website.