Ignorance isn't Into The Night's problem; it is YOUR problem. I will show you why in just a second.
A bird will typically display symptoms of infection. It might cough, it might sneeze, it might be lethargic, it might have discharge, etc etc... It's quite simple to distinguish a healthy bird from a sick bird.
You can't guarantee that other chickens won't get infected, but such separation of chickens typically involves grabbing the sick chicken and putting/keeping it in a different and contained area from the other chickens. A similar process can also be used on a broody hen.
There is no "miracle drug" to give them. Treatment usually involves separating them and making them as comfortable as possible (rest, food, water). The chicken might recover; the chicken might die.
Interesting. So bird flu is spread by migrating birds, yet "we" must "stop the spread" by culling entire herds of chickens (which aren't even migrating birds btw)? How does that prevent migrating birds from spreading bird flu? I suspect that "no poop zones" would work about as well as "no gun zones"......... Heck, maybe leftists should just "ban all birds" like they wish to "ban all guns".