Plain text messages being sent out as MMS instead?

Last Friday, I sent an MMS message with a photo to a friend. Since then, every message I have sent him has gone as an MMS message even though they're all plain text. I deleted our threads and started a new text message thread with him, but the problem persists. Has anyone else seen this?

I have gone into Settings and turned off MMS, which I think should fix the problem. I'd like to turn it back on, though, since I do use it occasionally.

As a side note, I also wonder if there's not some kind of virus on my phone now. That Friday, I received a spam photo via MMS (dirty picture!). My friend was with me at the time and thought it was hilarious for some reason, and wanted me to forward it to him - so I did. Then I deleted it. I wonder if there was something wrong with that particular message.