As a small-business owner, you may be missing key signals that your computer network has been compromised. Maybe there’s a sudden spike in traffic on the network. Or the system responds sluggishly. Perhaps some data are inexplicably missing or corrupted. Or suddenly the system is run ning out of hard disk space. Pop-ups may start to appear or Internet browser errors occur. Maybe there are no perceivable symptoms, yet plenty of mischief is going on.

Banks and credit unions across Virginia have sent letters to customers warning them that their credit, debit or ATM cards have been compromised. The financial institutions took the steps because of a data breach—believed to be the nation’s largest—linked to fraudulent activity at card processor Heartland Payment Systems Inc. in New Jersey.