I have played 1 and 2, watched both animated movies, and even watched some comic book like thing called No Known Survivors (I think) about the outbreak on the colony. None of them have explained how the first necromorph comes to be.
I know a marker sends a signal that drives people insane, they murder others and sometimes commit suicide, and that creates bodies. But how do the ray-like necromorphs (Infectors?) that transform the corpses into necromorphs show up? Are they formed in the growth on the walls? I haven't played Extraction, Severed, or that other DLC one, but I'm assuming they don't explain that either. Does anyone know how this works?
With each new installment, we learn more and more. At the same time, we always learn what we thought we knew, was WRONG.
I think this is what make the franchise so great. It continually keeps you guessing. I do know DS3 has yet another twist. It is set on an old Unitologist Outpost, where the Markers are thought to originate from.
I won't give you the full synopsis, but along the lines of your OP, there are now what are called "Feeders". These are stranded survivors of the outbreak that resorted to eating the necromorph flesh. They do not fully explain why yet. It was either an attempt to avoid starvation, or some twisted Unitologist ritual. The end result is zombie like creatures that attack you. They are not as twisted and mutated as the regular necromorphs. They look more like emaciated zombies with glowing eyes.
I honestly can hardly wait for DS3. I have played DS1 four times over now, and am in the middle of playing DS2 for the second round. This story line is fantastic, and I certainly hope it continues beyond DS3.
Sorry I don't have a definitive answer for you. I think only the people at Visceral know, and it is quite possible, they might not have come up with the answer yet either.
Just some of my thoughts. It is great to see there are fans of this franchise out there, ensuring it will continue.
That's one of the possibilities I thought of. But if the signal transforms corpses, why would the infectors be necessary? Other than the thing about them reaching corpses outside the marker's range. (this reminded me of something, see below) And I've seen a lot of screenshots and some footage of dead space 3, moblesuit75. There are a few changes that look interesting, including those people who ate necromorphs (necrovores?) I've gotta try not to see and learn too much because then I get too anxious about the game. I'll have to wait until its release gets closer before I look up any more about it.
If you've seen the animated movie Dead Space: Downfall, they explained there is a zone around the marker, yes the one form Aegis VII, that prevents the necromorphs from getting close to the marker. Hence "Dead Space". But obviously it was left out of the games. I think it would have made for some interesting gameplay when you had to drag the marker around at the end of Dead Space 1. A lot of things would have been different had they kept that idea. Maybe someone not involved with the game wrote that into the movie...