I simply don't accept that aircraft cabin air and environment are inherently any more "risky" than any other environment where human beings cluster together
Exactly.
On long flights, though, we tend to find ourselves in an area with more people per cubic metre (or cubic foot if you want to think in feet) than we find ourselves in the majority of our daily lives. Thus, when on long flights, we are more susceptible to germs, not because the air isn't filtered or is filtered, but because there are more of us in a smaller space. Even if the air is not filtered, but entirely fresh on every cycle through the plane, it has to come in somewhere and go out somewhere, and in between it goes past people.
I flew from Panama to the US a few years back, and there was a carry-on filled with mothballs in the overhead. When the flight crew opened the bag (because of the smell), the two of them fell to the floor immediately, several passengers passed out, the copilot got "dizzy" and the pilot put on oxygen and we did a "precautionary" landing in Mexico.
If the bag had been opened outside, and we were all gathered around in the open air, I doubt anyone would have been affected.
It's all about confined space, and an individual's ability to fend of germs. And as has been written already, there are ways to reduce your chances of being affected by other people's germs, follow safe practices and you're less likely to be affected.
I haven't generally been affected on flights, but my son, who suffers from asthma always used to get some kind of bug on long flights. We started taking precautionary measures with him (bathing before and after the flight, hand gel, masks, etc.) and he was no longer affected.