how often do you replace your hens?

[Semo Farmgirl]

I bought Buff Orphingtons from a hatchery knowing they tended to go broody and they were good mamas. Last summer they hatched 7 babies! I gave the roosters to a friend and kept the girls. At about 4 months they got sick just one at a time. Could not stand, neck got limp and I separated them off. Held them and gave them electrolytes worked hard to get them to drink and swallow. All of them recovered but one. A few months later they started dying off one at a time. None of my older birds ever got sick. It makes me not want to raise my own babies. You can’t vaccinate them yourself and I’m not sure what was wrong in the first place. I thought Merrick’s Disease but they didn’t have all the symptoms and the organically survived and all my adult chickens did not get sick??? My adults are now two years old. Any advise?