What is the thumbs.db file?

Anytime thumbnails view is enabled or has been enabled in Microsoft Windows the hidden file thumbs.db is automatically created in the same directory as where the thumbnails have been viewed. This file contains the information required by Windows to display the thumbnails for each of the icons and will be placed in every folder thumbnails are viewed.

Can it be deleted?

This file can be safely deleted from any directory, however, it will be automatically recreated if thumbnails view is still enabled and you view that directory again.

Is this file a virus?

No. This file is a Windows system file. However, like every file on your computer it too can become infected. If you're concerned about a file being infected on your computer use an antivirus to scan the file.