Once mastered, open Mail and open the Mailbox option in the menubar.

Choose Rebuild and let it do its thing.

On completion, close Mail and reopen it.

Did the problem go away?

On the subject of a virus. There are no viruses that are effective against OS X, but it is possible that there is a Windows virus attached to one of those emails.
However, even if there was, it would not do anything against your Mac.

P