Wow, thanks folks for all the good advice! I think what this means is that I'm being overprotective, not only of my chickens' health, but my own (I have asthma, so when the dust builds up in the coop, I start wheezing). I got all the chicks as babies from a feed store, and perhaps that was a bad idea. I also get my feed from the same store, which I've always assumed was good for them. In addition to their organic crumble, I feed them a bit of scratch every day, meal worms because they absolutely LOVE them, sometimes some fruit, greens from our garden, some plain yogurt with good bacilli, but nothing greasy or sweets. Lately, they've been feasting on slugs because we've had relentless rain, and will eat any other bugs that come their way.
It's strange that all their problems are different. The sick one now, I think, has respiratory problems, which is not surprising given the wet weather. Out of 6, I have maybe 3 that are healthy--so far. I lost one (the 7th) at the beginning, and 3 others that are always sick. I have a problem knowing when to put animals to sleep, and they definitely are my pets, not livestock, so that makes that decision more difficult. Even though I clean every day, I don't ever clean out the whole coop and runs. What I do is every day, clean the roosts with organic cleaner, pick up all the poop from the night before, wood shavings in the coop, straw in the runs. I'm sure there's mold in the runs because moisture gets in, but it does have a roof; a wire fence otherwise. I also dust all the windows and surfaces, to get rid of dust in the air.
If I knew how to attach a photo here, I'd show you a picture! Thanks for your advice--I knew it wasn't right that chickens were this much work!