Organisers have announced that the event's poultry competition has been called-off after a vet discovered a case of infectious bronchitis.
A spokeswoman for the show said "birds under suspicion" had been removed but it was with "great regret" it has had to cancel the poultry competition.
Roger Thomas, who had birds that he was showing, said it was "disappointing".
The show, the largest agricultural event in the West country, started on Wednesday and runs until Saturday in Shepton Mallet, Somerset.
Alan Lyons, head of shows, admitted it was disappointing but said there had been a "great day judging on the Wednesday" with "record entries".
"It is very disappointing for the show and all the competitors in the poultry show but we take our bio-security measures very seriously," he said.
Infectious bronchitis, according to the Department of the Environment, Food and Rural Affairs, is an "acute, highly contagious" viral respiratory disease with "high morbidity".
Up to 1,200 birds have been removed from the Royal Bath and West Show due to a suspected outbreak of disease.