Hen to egg would she be thinking of Mycoplasma Gallisepticum ?
Depends on the symptoms she has, but MG is transmitted hen to chick via the egg.
MG presents itself around here every so often it seems in waves, where we see alot of it.
Other years we do not seem to have any issues of it at all.
Spread mostly by migrating fowl, I am guessing, but it can & is passed to the chicks via the hen, in the embryo.
http://en.wikipedia.org/wiki/Mycoplasma_gallisepticum
Could be. Both are relatively minor threats that are all over the place and most long time breeders dismiss as irrelevant. You're right about the migratory birds, and then the native birds spreading it. Never understood why people make such a huge fuss about biosecurity and how dangerous shows are for spreading disease and then let their birds free range, or don't make their pens rodent and wild bird proof. I get either approach, but you can't have one and not the other.