Natural remedies to boost rabbits immune system.

Ok, so I know this may not be the best place to ask about things for rabbits, but I was wondering whether someone may be able to help?

Anyway Drew has before been to the vets, and the vet has said that he has had pasteurellosis, he was prescribed baytril, and I was told that it would more then likely re-occur throughout his life, now yesterday I went to pets at home, ended up spending way too much, anyway moving on, whenever I go there I pick up burgess hay for the pets as a treat, they normally have a pillow wad of meadow hay I think its called, anyway, I read the packet and it mentioned marigold, and how it boosts the immune system today, and it got me thinking, does Marigold actually help their immune system? And could having things that naturally boost the immune system stop the pasteurellosis coming back? And if so, what else could I give him, natural things, would help him?

Anyway it may be a longshot, I don't know whether marigold might help the immune system, or whether it's a advertising ploy, and I don't know how much research is put into things like this, but I'm planning on looking into these things more, but does anyone have any tips or advice on what to look for? Or know what does or doesn't work?