Coccidia are everywhere, and it doesn't always need to be wet. Chickens usually develop resistance to the coccidia in their local soil if they're exposed to it from early chickhood, but adult chickens brought in from another locale can be susceptible. Also, if you bring home top soil or sand, that can import a different strain of coccidia and your chickens won't be resistant. Even loaning out a mower and when it's returned, mowing your grass can import new coccidia. I recommend always keeping Corid on hand as coccidiosis is one of the most common ways chickens can get sick.
Corid is safe. It's is not anything that interacts with other meds. It can save a chicken's life. There is a very inexpensive test any vet can run for you. Gather a fecal sample from a sick hen and have a vet run a fecal float test. At the same time, the vet can see if there are intestinal worms as well as coccidia in abnormal amounts. Worming can be safely done at the same time as Corid.
If your husband is adverse to trying treatments without knowing for sure what is making the chickens sick, tell him you can wait for the chicken to die and then take the body to an agricultural lab and have a necropsy done. It will tell you what killed the chicken.
The cost of trying these treatments is not nearly the cost of a necropsy. And you just might still have a live healthy chicken when it's all finished.