Im wondering what the answer is for that..My cousin has cert organic farm..they raise some chickens no medicated anything and never a problem, maybe its some of the breeds they raise, not sure..very clean environment, 200 acres lot of outdoor , they feed all certified organic feeds.move chicken tractors around every day...I think they select a bit for immunity..I know the birds with mediteranian blood are most part immune to Mereks ..they carry a B factor in thier blood..I do combo but I think as Vickie says..keeping them clean of parasite load and good feed so I do use commercial wormers , but prefer to find feeds that are very low in soybean..soybean is a natural hormone in medical feild causes very young girls as young as 6 yrs old to begin menstration and breast development..for a long time they couldnt figure out why this was happening..then med research found that the soybean mimics natural hormones produced in the body ..they began adding it to cerals, and lot of store shelf food..may play a role in breast cancers ect..but not been proven..it is suspected as it would be hormone overload..