Here's a brief synopsis of a few treatments for those puzzled about how to treat for such things and still have eggs and meat you would want to eat.
There are a few treatments that you can keep in your arsenal that can help if your flock gets mites but I can tell you that we kept chickens for over 30 yrs before seeing a mite and only then because we got chickens from a place of poor husbandry~so don't get too troubled and start thinking you have to do all kinds of preventative measures to not get them....you may never have to deal with mites and lice in your flock husbandry. When I got them I had to find something that could help them that was in the realm of natural, so here are a few things you can try:
Castor oil and NuStock for leg/scale mites, wounds, worms, fungal infections: Both are effective with just one treatment, in most cases. Both are comprised of all natural ingredients that are not harmful and only beneficial. Of the two, I am impressed with both...but the castor oil also can be used for deworming, if you so desire, as well as an antibacterial and antifungal treatment for wounds. I've never had to deworm a flock in all my many years, so that's just an option if you need it. The Nustock is good for wounds, fungal skin infections, hot spots on dogs, rain rot on horses, mange, etc. and is comprised of sulfur, pine tar and mineral oil only.
Dusting for lice and mites: Wood ashes help, sulfur dust can be found in any garden department and can be used to treat roosts, bedding and nesting material, as well as the birds and is effective as well. Some use lime for dusting the birds and bedding, as well as walls and roosts. If none of these work and you have a persistent case, Pyrethrum is a natural substance derived from the chrysanthemum flower that is very good for this. Do not confuse it with Permethrin, which is a chemical preparation that is more harmful to the environment, the insects and the animals in your care...not a good one to try, in other words. I don't use DE because of it's ability to harm beneficial insects as well, though I know many throw DE around like it's money, I never recommend it. Another natural option is plain ol' kaolin clay...let your birds dust in it and treat themselves.
Worms: Castor oil is safe for humans and animals alike and has been used for centuries for this. Raw pumpkin seeds contain cucurbitin, a chemical that can paralyze the worms until they detach and are flushed out of the bowel along with the feces. Ginger root is another natural antihelmintic, as is garlic. Simple soap in the water acts as a surfactant and helps to dissolve the oils that protect the skin of worms, allowing them to be killed by the digestive acids and enzymes in the bowels. Black walnut hulls, while still green, are used for deworming. Charred wood has been used for this as well and one can flake off the char and add it to the feed mix....for other livestock, just place it in their pens and they will gnaw the charred bits off the wood.
The best treatment of all is to use preventative measures such as providing good dusting opportunities all year round, clean soils underfoot by providing free range, well managed deep litter in the coop to encourage beneficial microbes underfoot and predator bugs that prey on mite larvae, feeding and watering indoors where vectors such as wild birds, rodents, etc. cannot access feed and water. Treat roosts and nesting boxes if your area is prone to this problem, but not the bedding and the bird unless you actually HAVE a problem. Feeding fermented feeds or adding mother vinegar to the water can create a hostile environment in the bowel of chickens that can help prevent worm infestations, but will not deworm a bird already infested.
One very important tool that no one ever mentions and that is yearly culling for health, performance, conditioning and appearance and feed thrift. Culling for these traits can naturally eliminate the birds that carry parasite loads due to poor immune system function and old age, while also preventing problems like egg bound, internal laying, prolapse, etc. Breeding chickens that are naturally parasite resistant is also an important step.
Avian biologists claim that 90% of the flock's parasites are being carried by 5% of the flock, so by eliminating those 5% of birds in a yearly cull by targeting the traits of a bird carrying heavy loads of parasites, one can keep problems like this down to animals who thrive well with an acceptable load of parasites and also breed for more of the same.
There are other all natural treatments for these things if one wants to dig, but these are the most commonly found and some of which I've actually used and can attest to their efficacy.