I agree, there is no advantage at all to living like a "Germaphobe", not for people, not for pets...It's been well-known for a long, long time that living creatures build-up immunity to Bacteria, Fungi, Viruses, etc. by simply being exposed to them. And it's also been know for a long, long time that over-exposing a living creature to the same Antibiotic over and over again makes them useless when they actually might need them...I don't believe that there is any documented evidence, not that I've ever read or heard of, that keeping pet birds in a "sterile" type of environment gives them any advantage as far as not contracting illnesses. That's just not how it works...
I'm 38, I got my first pet bird at the age of 6, and have owned my own pet birds as well as breeders since I was 16. And with the exception of my Quaker developing a bacterial Upper Respiratory Infection about a month after I brought her home from her breeder (which means she most-likely contracted the infection while she was at her breeder's home and while with her siblings and the breeder's other birds), none of my birds have ever been ill either...They've been injured due to accidents before, but none of them have contracted any type of infection, at least not one that I was aware of and that their bodies didn't fight-off on their own...
And that a really good point, the fact that our pet birds, just like any of our pets and just like us, probably do at times contract illnesses/infections, but because their are in good-health and have strong immune systems, their bodies fight-off the illnesses/infections without us ever knowing about them.
While it's easy to identify the causes of illnesses in birds that are very specific in-nature, such as heavy metal poisoning, or death from the fumes of Teflon, or Fatty Liver Disease due to a fatty diet (though we also have to account for other things like genetics when trying to figure out why one bird on an all-seed diet gets FLD and another one on the same diet doesn't), what is nearly impossible to know are the causes of sudden infections, such as an Upper Respiratory Infection, which is usually caused by either a Bacteria, a Fungi, or both...It's one thing if the bird is immuno-compromised and then develops a URI, but when a perfectly healthy bird develops a URI, no matter what the microbe is that has caused it, usually it's impossible to identify the cause (unless the bird was exposed to another sick bird, of course)...It could be anything, just like it is with us...What we do know is that birds don't get infections (not Bacterial, Fungal, or Viral) just from being out in the cold, neither do we, that's an old myth passed down throughout the years...Yes, if a bird is left out in the cold or is wet and cold for a long period of time, just like a person, they are going to get sick, but not initially from an infection. Birds in that situation only develop infections after their immune systems and their bodies in-general are weakened by the exposure/hypothermia, drop in body-temperature, frost-bite, etc. So any bacterial, fungal, or viral infections that develop in a bird, reptile, or a person who has been out in the cold, wet weather for a long time develops "secondary" to the exposure to the cold/wet weather...
The bottom-line is that for any living creature to develop a bacterial, fungal, or viral infection, or any type of illness caused by some kind of contagion, they must be exposed to that specific contagion in some way. That's it, period. However, why does one bird exposed to a certain contagion become ill while another bird exposed to the same contagion is not effected by it at all? Genetics, strength of their immune system, one having built-up an immunity to that specific contagion while the other has not, one on a healthy diet with a stronger immune system as a result, etc.
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