The Immortals

[Sean Manning]

It can be, following some iranists, simply a mistranslation of Herodotus or more likely, of his informer;
the old persian word for "Immortal" would be "anausha", and the word for "follower", "anushiya", so, the informant mixed the two words, and the elite unit would be simply the "followers (of the King)", the "loyal", most probably the iranian people in contrast with the other nations and peoples in persian army.
So it is posible that there would be no "immortals" at all never, and the tale of replacing men after they fell was a folktale-like history created after the greek name of "athanatoi". It makes sense, however, other iranists have also reservations about this theory.

So the original name could have been 'Anushiya', which could then mean 'Companion'? Well, that's a name that is a normal one for the elite troops around any king, right? Makes sense.

And we know that Persian nobles had large groups of household troops who travelled with them and ate nearby. I'd forgotten about that theory, but it makes sense.

Jona agrees with that theory: http://www.livius.org/ia-in/immortals/immortals.html