I think that's it. It says it can be caused by excessive humidity, and I had humidity problems the first day or so. It also says it can happen in cracked eggs; they all arrived intact, but some of my hatched chicks chipped a little piece off that shell yesterday. I wonder whether that let the infection in. There was nothing visibly wrong with the chick. I had a 'miracle chick' yesterday. It was struggling to get the shell off its butt, and I made the mistake of pulling it. There was an awful lot of blood coming from its butt, and it just layed there so I thought it must be dead. I set it in a box to take outside, and went to wash my hands, and when I came back it caught my eye and I saw it was breathing. I set it back in the incubator, and when I got up this morning it was cheeping and walking around.