I found this on-line:
Cryptosporidium cuniculus—The rabbit is an intermidiate host of this protozoan. Cats are the definitive host. Rabbits in households with cats can potentially jump into the cat’s litter box, step in infected cat feces, and then ingest oocysts when cleaning their feet.
Infected rabbits may have no clinical signs, or may have fever and die within a few days.
The infection is treated with parasiticides. Rabbit owners who also own cats should be very careful to keep the cat’s litter box immaculately clean, to wash their hands after cleaning the litter box, and to keep the rabbits from using it, if possible.
Here’s the link:
http://www.gopetsamerica.com/small-animals/rabbit/internal-parasites-rabbits.aspx
It’s further down at the bottom of the article.
Henri probably would not like the cats using his box either. I have no ideas how to prevent this – hopefully some other members will have good ideas.