IDK but I left it out too

Whether it is strictly necessary or not is a question of accuracy that I can't give a sure answer, but it is necessary to sound smooth （顺） which is important when deciding whether or not to leave some characters out or add others in.

It would sound at least a little unnatural to leave 在 out in this sentence. You can, perhaps, think of 生病 as a verb, not a noun, here.