Sneezing and wheezing chicks

[Sammy3]

Hi there, can anyone help me. I have a dozen 7 week old chicks that are sneezing and wheezing quite badly. They are indoors at night and under a lamp still in a large shed during the day. They have been gradually sneezing more and more over the last two weeks and now one of them is very sick and I don't think she will make it through the night. From what I have read they are not old enough to have worms and they are healthy in every other way, eating, drinking, running around, clean bottomed. They have not been out in the garden yet so cannot have picked anything up. Can anyone tell me what the problem might be? Im worried they are all going to die.Thanks, Sammy