All the cats have to be treated. even though you haven't seen worms in the stools, dead ones go undetected and eggs are not at all detectable. Sharing a litter box means sharing roundworm eggs! (they wash their feet after walking on eggs). It's easy enough to treat though! Don't let a flea infestation take over! I had 2 summers of treating fleas and the whole apt. and it's a pain! (ringworm fungus is worse - well, harder on the cats and so time consuming but fleas are also expensive and time consuming- it's the eggs that hatch in 3 weeks that keeps you running and cleaning the floors and upholstery!)
When I used to do volunteer work at a local cat shelter, one of my co-workers witnessed a cat (stray brought in by QueenScoopalot- no less!) vomiting up a huge ball of live round worms! She said it looked like a bowl of moving spagetti! Retch, retch!!!! Apperently when they are ate the point of vomiting them up is when they are really badly in the cat's system.
Let us know how they do (and you with this too!)