What is iframe and how is the best way to find the code?
Time for a few hours of Google search and catching up on the basics. iframe is html code. it allows one to "frame" content from one source and show it at another location. Long story short, you could show content from my site on your site using an iframe. When the frame size is set to "1" by "1", it becomes invisible to the naked eye.
Can I ftp the site to my desktop and run a virus software
If it is an iframe, it is only pointing to a malicious location, and, in and of itself, it is not a virus. For most purposes, that is probably ineffective. But I won't go so far as to say that scanning your files for anything that may trigger anti-virus is a bad idea.
Should I search for a text string with 'iframe'.
That may be a help, if you search the posts and comments tables in your database, it may pop up, if it is the problem. Not all iframes are bad. But people are usually well aware of the legit ones.
I'm afraid at this point, you have some reading to do, and I am quickly approaching the limits of my knowledge on the subject. I hope you find the culprit.
Good luck to you.