I feel we have to put into the big picture many of the 'dangers' out there. Pathogens are everywhere. Like Amy said, germs <b>can</b> live on surfaces for days, but these bugs also come from just about <b>every</b> conceivable environment we can possibly imagine and we can't eliminate everything. We can take reasonable precautions but I think we can probably make ourselves crazy over this.
In my non-expert opinion based on what I read, Philip would probably be exposed to more aspergillus simply by spending time in an unfinished basement (which he isn't, anyway) than by being near a sanitary bird cage that happens to be residing in it. (And that isn't an insult against your unfinished basement, btw- it's just the nature of any typical unfinished basement to be dusty and more damp than what the rest of our house is- both environments in which aspergillus thrives.)
But when it comes to these kinds of worries; we don't have birds, but I won't keep out of the garden and I won't stop Emily from helping me nor will I stop her from playing freely outdoors, which are other places this stuff shows up. Will I catch flak for that from a few here? Maybe. Will it take away from Emily's qol to not be able to play outside w/ her brothers and enjoy spending time with me outside doing something I love? Most definitely. It's not worth it to me, since she will most certainly be exposed to those pathogens (aspergillus, pa, etc) in multiple other environments anyway. But it IS very hard to always know what to do and hard not to worry; it's the nature of things as a parent.