Its users Microsoft Security Essentials report that they can no longer get updates about new viruses and it seems to be a new bug in Microsoft software.
Online reports that while the automatic update option is enabled, it stopped receiving updates a few days ago and if they click on "Update" it still does not work. Users have no choice but to download and install the latest malware updates themselves.
The bug was first reported on January 19, but Microsoft has not yet released a solution or, at least, made a statement on the issue.
The solution is to download them standalone updates for new viruses. Try the Microsoft official page.