What happened to Kiel Martin late in Season 4?

Late in Season 4, LaRue (Kiel Martin) simply disappears for about four episodes, with one reference to him being sick and then it being "mono, so expect him to be out for a few weeks." He remained in the lead credits, appears in the season finale and of course continued to the end of the show.

Detective Harry Garibaldi (Ken Olin) is propped up in his place and becomes a continuing character, but seems very much like a cardboard cutout in these shows and delivers a lot of dialogue that sounds like it was written for LaRue.

What happened to Martin in these weeks? Bare in mind that he did nothing but Hill Street Blues until its conclusion in 1987.