In this case the virus was in an attachment named document2174_pdf.zip. This filename was unique enough that I was comfortable searching for all instances of that attachment and deleting the messages. If the filename was more generic, I would have included the message subject in the query.
Before you can perform this type of search, your user account must be a member of the Discovery Management group. This is required to do multi-mailbox search. Group membership does not take effect immediately, you may need to close and reopen your Exchange Management Shell prompt.
The syntax I used to delete all messages with the specific attachment from all mailboxes was:
Get-Mailbox -Resultsize Unlimited | Search-Mailbox -SearchQuery "attachment:document2174_pdf.zip" -DeleteContentNote: This deletes the entire message, not just the attachment.