In response to the above comment on natural remedies etc, when you are sick you go to the doctor, they will usually prescribe you a medicine to make you better. Why should it be any different with a chicken.
Because man made pharmaceutials frequently have side effects which can be worse than the disease they treat. Is why.

Whereas natural ones don't have these side effects. Nobody does this natural 'gig' for the fun of it, let me assure you. It's a choice born of necessity. And no, when I am sick I do not go to the doctor. I use natural remedies. Many people do. I can personally say garlic's saved my life. I've been a skeptic too, I've exhaustively tested the remedies I recommend. I didn't accept anything at face value, and while I value science, I know to take it with a pinch of salt. Often a pretty large one.
There are entire cities of people who use natural remedies exclusively, in fact in some parts of the world it is still accepted as being the best way, and in parts of the world that use man made drugs, there is a growing push by patients and doctors alike to return to using more natural methods. Some doctors are behind the times, but more and more modern research is emerging which proves that medicine should be holistic; treat through diet as well as with any surgeries needed, etc... Not just pharmaceutical drugs. Modern medicine has its place and potentials, even though it's taken many a wrong turn as probably every field of medicine has in its time; but natural medicine has an even more important place because if an organism is fed correctly, most disease cannot take hold.
I'm not advocating just going cold turkey on normal mainstream medicine; notice I did make allowance for antibiotics possibly being a quicker fix choice. It's the side effects that I wouldn't want to have in my animals. Some antibiotics permanently destroy intestinal flora and fauna that are necessary to living a long life. Some harm the liver or other organs, often permanently. The natural sources of the drugs used is gentler on living organisms because in its natural form it arrives in the correct form and in the company of all the other vitamins, minerals, enzymes, acids, etc needed to process it correctly; once you separate it and process it by most of the methods used, it is no longer safe and acts differently in the body. It's as simple yet as profound as the difference between raw milk fresh from a cow being fed to a calf, and raw milk fresh from a cow being boiled then fed to a calf; the first way it thrives, the second way it dies.
There's a lot of new studies coming out in the news and mainstream media proving natural remedies weren't just 'old wives' tales' --- like, for example, garlic is more powerful an antibiotic against many bacteria and viruses than man made antibiotics; it can kill bacteria and viruses that they can't touch. All without side effects. It contains Allicin among many other natural dynamic antibiotics and other compounds which make it a powerful medicine. It's not known as 'russian penicillin' for nothing. It doesn't allow immunity to build up because the active antibiotics are formed differently every time from the enzyme reaction of crushed or cut garlic, whereas stable, unchanging man-made antibiotics are not only quickly irrelevant to viruses (etc) but help develop even worse, stronger 'super bugs' as I'm sure you have already heard. And they also have side effects in the body which can cause permanent harm.
Also, all herbs and plants will contain drugs, pharmaceuticals are just concentrate versions of many of these drugs that are found naturally.
No, they're rarely just 'concentrate' versions, they are usually synthetic and contain additives, and if they are just extracts they've almost always been over-processed or otherwise unnaturally altered. They are no longer just 'concentrate' versions. Our bodies (and our animal's bodies) cope with the natural version but can be damaged by artificial versions.
I am not against using things like ACV and garlic, or yoghurt to improve a birds immune system as long as a bird has also been treated for any diseases or problems with proper medications.
Natural medicines can and do treat diseases and problems; they are in fact proper medications by definition; I'm speaking from experience in this matter. I often treat condemned animals successfully, after 'proper medications' have failed. But it's harder to treat an animal that's already been treated with 'proper medications' because they do so much harm as well. Much easier to treat an animal that's been reared with natural methods.
Pharmasutical companies spend millions to make and come up with these medicines, and to prove that they work, so why would anyone not use them when they are needed.
Just think about that for a minute... They spend millions to make them and 'prove' that they work. Big business. Of course they are invested, and it's not in living creature's health; they are invested in our illness. Please, dig a little deeper, check out their studies that were used to determine that they 'worked'. What you will find will shock you. They word these studies like an incompetent lawyer, with surprisingly transparent doubletalk, white lies and implications, because of those millions you spoke of, which are actually billions, probably trillions by now; they have huge advertising power and advocate drugs known to cause all manner of horrific side effects including psychosis in kids and newborns, and the likes. Treat the common cold! Side effect: insanity. That one was all over the media recently. That one was proven. It's not alone, there are many examples. But their advertising prevails. Surely you know someone who takes high blood pressure medication; please, read the label and google the side effects. Shocking.
You can access the drug maker's own websites where they will tell you better than I ever could the actual effects of their 'proper medications', their drugs which you trust. Don't believe me, by all means, find out for yourself! Don't bother to listen to their ads, though, obviously that's propaganda. Read the fine print.
When pharmaceuticals first started it was more or less lawless and they would hire any 'scientist' to make 'studies' proving natural remedies, their biggest business competition, didn't work. They would resort to all sorts of desperately biased and skewed and deliberately falsified studies to 'prove' this so they could advertise it, get nations to stop treating disease the way they always had, and start using expensive pharmaceutical drugs and concoctions instead. You can access some of these old studies for a good laugh. Modern ones are just sad. We have a disease driven industrial system that thrives off our suffering. It's all profit driven, not philanthropic. Many of the professionals in it are indeed well intentioned. But when these times become history, future people will be shocked at our barbaric 'modern' medicine. All through history, every civilization thought its science and medicine were perfect or near so. As always, time will tell and future generations will laugh at past ones. Our time is no different.