I read that the most common cause of salmonella infection in flocks is from rodent droppings in the feed. This could happen to any of us, too, if we don't keep our feed in a secure container.
http://www.accuweather.com/blogs/news/story/35787/iowas-wet-summer-may-have-help.asp
http://www.accuweather.com/blogs/news/story/35787/iowas-wet-summer-may-have-help.asp