"AT&T has received information indicating that one or more devices using your Internet connection may be infected with malicious software. Internet traffic consistent with a malware infection (“unknown”) was observed on Jan 20, 2018 at 7:45 AM CST from the IP address (my IP addy) Our records indicate that this IP address was assigned to you at this time."
What a weird message. Traffic consistent with a malware infection, and yet an "unknown" one.
"... one or more devices using your Internet connection..."
I don't know much about networking. I wonder if someone nearby is using some of your bandwidth, and the "infected" device is THEIRS.