Comcast is launching a trial on Thursday of a new automated service that will warn broadband customers of possible virus infections, if the computers are behaving as if they have been compromised by malware.

For instance, a significant overnight spike in traffic being sent from a particular Internet Protocol address could signal that a computer is infected with a virus taking control of the system and using it to send spam as part of a botnet.

Comcast is launching a trial of a service that...