I have a flock of 20 hens. It seems that every once in a while one starts to sneezing and coughing and then another one will get it. Somtimes they act listless and sometimes not. Sometimes they appear to get over it but it takes a while. My question is should I get rid of all of them and start over? Do you think this is in the evironment or is it passed bird to bird. There living quarters are some what clean. No bad ordors. Any help would be appreciated.