You can keep your chickens healthy using all natural approaches if you are consistent and diligent in your husbandry practices. Sentiment cannot play a part in keeping and maintaining healthy flocks. I've never had illness in any of my flocks over these many years and I don't believe that is just good luck...it takes observation, prevention, and intent.
I use unpastuerized apple cider vinegar in their water as an immunity booster and for the good vitamins therein. I never use antibiotics, chemical dewormers, medicated feeds, vaccines, etc. I free range at all times in an acre lot with farm dogs on duty that prevent predation. I feed a mix of layer mash(no animal byproducts, thank you) and whole grains such as oats, wheat, BOSS. I do not continuously feed as this is a waste of feed and attracts rodents.
Another method of building a good immune system while keeping your birds warm in the winter? Deep litter with attention towards keeping the bedding fluffed and dry....your birds can help you with this by scratching for BOSS in your litter.
I've never dewormed a flock except to supplement their forage and feed with pumpkins. I've never disinfected any living areas, never used DE or other dusting powders to prevent lice or mites, clean out waterers with a swish of the hand and fresh water ~and maybe a bleaching once or twice a year.
Nu-Stock is my flock treatment of choice for scale mites (had those once from donated hens) and any wounds they might incur. Nu-Stock is merely sulfur, pine tar and mineral oil....works in one treatment on scale mites when all other natural treatments failed. And the mites did not return.
You can use liquid garlic juice or powder in their feed or water also.....great immunity booster and helps ward off blood-sucking parasites.
Your most useful tool for a healthy flock? Your brain. Choose breeds that are known to be hardy in your climate and in general. Choose individual chickens who thrive on your animal husbandry practices and in your climate....all others need to be culled from your breeding plan. No exceptions. If a chicken always looks raggedy or thin and you have no explanation~cull her. If you have one that insists on leaving the flock and roaming into dangerous areas~cull her. If you have hens that are victims....squat too often for breeding and have a constant bare back...cull them. Bare skin is a source for possible injury and infection...best to cull the birds who never look fully feathered, bright eyed, active and lay well.
Ornamental or bantam breeds are cute to look upon but may not always be the wise choice for your flock, so resist the urge to go with pretty, unusual or weird and keep with the natural beauty of standard breeds with a proven hardiness.
Cull for inconsistent laying practices in peak laying times. If she doesn't lay every day or every other day in peak times, in low times she is eating you out of house and home. Cull any birds that are always fat and don't lay consistently...these too are wasting your money and a too-fat hen is prone to reproductive problems like becoming eggbound, ovarian cysts, reproductive cancers, etc.
Do not use supplemental lighting or heating for your adult birds, try to raise chicks under a broody hen, expose your birds early to your soils, the weather, the climate changes you have. They will feather quickly and build a healthy immune system that doesn't require meds to keep healthy.
By the time you are done with this multidirectional approach to a healthy flock, you should have a flock anyone would be proud of.