I'm a firm believer that if you have worms in one, you have worms in all; Chickens explore the world with their mouth, tasting everything including each others excrement. The severity of their exposure may differ. Since your birds have never been wormed, I'd certainly err on the side of caution and de-worm them all. It's a simple process really, the feed store sells the medicine in packets that you dump into ALL OF THEIR WATER SUPPLIES. They should not have access to untreated water for the course of the treatment. You will treat their water for 3 - 5 days according to the directions of the medication you purchase, do not supplement with vitamins during these days as some of them work by BLOCKING vitamin absorption. After the course of the treatment is complete, put some yogurt into the chicken's feed for a couple days to ensure good gut bacteria. After a week since the treatment, you may reexamine if your worming solution covered coccidiosis as well, most do not. If not, now would be a good time to medicate them for that; several options are available for treating coccidiosis, everything from tablets, drops to more powder to put in everyone's waterer(s). I really prefer the power in their water method. Again, if one has it, they all have it, and it won't hurt them to prophylactically treat them for this disease. I go through this entire worming process 2 or 3x annually as my birds free range (in my HUGE fenced yard) and I have been known to feed the occasional raw-meat treat when I feel they could use a protein boost, and they have open access to any fruit that falls off of the trees onto the ground (an excellent source of worm and worm egg ingestion).
