[COLOR=FF0000]I don't think that I agree with your assessment above.....Here is something to think about.[/COLOR]
When you have a flock that is mostly thriving and one is having issues....even parasites....
there is usually an underlying issue with that particular member that is making them weak and not able to fight the infection, parasites, or whatever else may be the case.
[It also happens with people. If you can wade through the CDC statistics over the years, you'll find that most people who die after getting influenza don't die due to influenza. They die because they had an underlying issue and their immune system was not able to fight off the influenza like a normal, healthy person could. So they end up dying, but not due to the flu, but due to the "complications" (as it is so nicely put) of the flu.]
If you think in terms of animals in "the wild" with all the same living conditions, exposure to parasites and various illnesses, etc., some of them are able to "fight off" those issues with their own strong immune systems; others are not and die.
In a "wild" situation, the remaining animals are a strong population that are more likely to pass their immunities down to successive generations, creating a healthy group for the future.
Nothing was different for these individuals except that perhaps genetically they were better equipped to fight off the things they would encounter in their lives.
Many "old time chicken flock keepers" feel that if you want to have a strong breeding stock in your flock for the future, you should not try to do "heroics" on a bird that is not thriving. Or at the very least, not to raise birds from their eggs. They see it as a "natural culling" so that if you continue to breed your flock, you are breeding strong, healthy birds in the future.
So...My "take" on it is that if the rest of your flock appears to be thriving, this particular bird most likely has issues of the immune system or genetics that made it harder for her to recover from the normal things that she was exposed to.
Just to clarify, I will say that there are things that you can do to make them as successful and healthy as possible. But if good husbandry and feeding is being practiced, and the rest of the flock is thriving, I also agree with the old time keepers and see it as a "natural culling" of a weaker member of the flock.