John Harris was taken ill during the US tour 76 (I believe) John was employed by Queen as a permanent crew member. Queen had hired a Clair Brothers rig for the tour so they were approached to find a short notice replacment to finish the tour.

I'm not sure what the illness was but I know that the recovery time was well over two years. In the mean time Trip became their go too guy.

Harris, by the crazy tour, felt he wanted to return. He did the tour but my understanding is that he no longer enjoyed being on the road and retired from live work after that tour