>> reporter: because that answer didn't quite square with the speeches, the promotional videos and the under writing of his pbs specials, cnn called the ben carson campaign asking for an interview. the response from the campaign's business manager quote, this will never happen. you are up to no good. we don't trust you. last week, we approached the candidate himself at a book signing in florida. dr. carson -- without any hesitation we were invited on his because for a two-minute interview. >> we're here to talk about a subject your campaign manager says you don't want to talk about, this mannatech thing. given your stature in the medical community, why were you involved at all, whatever the relationship was in a company selling a product that the texas attorney general says is sham product? >> well, remember, they contracted me to give a speech. i do not, you know, go into great depth when i get a contract to do a speech. i go and do the speech.