Double tap on your home button to bring up any open apps. Touch the minus sign on the mail app to close it, then return to your home screen. Next go to settings on your iphone and touch Mail. Touch the account that's having the issue to open its email settings. Swipe Mail to "off" and let the phone cycle through the off process. Once its complete, swipe back to "On". Your email should be set back to 0.
go to settings, then to "Mail, Contat, Calendar" option
Select the acount you are having issues with
Change the "Mail Days to Sync" to "1 DAY"
Now go to to Mail app and sync your account again. Unread option should go away
Now go back change the "Mail Days to Sync" and change the option back to what it was.
This should fix this issue. This has worked for me.