eperdemic / pandemic viral out breaks , air bourn bacteria, faeces ,contaminated water,poor air quality all these reasons not to keep birds overcrowded most diseases are from bad management and poor obsevation granted but in my opinion if you have too many birds in one area then your asking for trouble and if people want to house there birds like this i hope they know a good vet because any disease entering the loft will run straight through the flock , in the past i have had yb sickness and had it for 2 years last 8 years clear, as you say let it go through your flock as they will suppress it and get over it with a little help and will illiminate the weeker pigeons with a poor constitution. Too many pigeons will also result in them getting respiritry due to bad air , and for the fancier psitticosiss or pigeon lung ( newcastle disease ) . Keep your birds as you want , overcrowded lofts make a god haven for poorly underperforming pigeons. Good debate , ill think ill have to agree to disagree . ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D