Be informed that most disease causing microoganism thrive best in wet condition. it is important then to observe strict hygine over the cold season that favours the diseases. you can start by ensuring that your have fresh litter in your coop by staart of may and have it replace every month to avoid microganism built up(major calprit being coccidia and polurum. have your birds on multivitamin suplimint to relieve stress and boost their immunity