one died yesterday and another one today. They were both fine one minute, tired, lethargic looking the next, then dead an hour or so later. I separated them eahc when I'd noticed something wasn't right. Put them back into the brooder to keep them warm and separate from the others. The group of 15 are outside in a pen with a shed to sleep in. It's almost summer here so it's not cold outside at all. What happened? What can I do to prevent my other chickies getting sick?