here is Opin88's awesome theory on how halfas are created. on a scientific scale.
First of all, a person has to have two things killing them at the same time. With Danny, it was both electrocution and ectoplasmic radiation. For Vlad, it was probably something that he contracted while in the hospital coupled with ectoplasmic radiation. The key is the ectoplasmic radiation. My theory is that raw ectoplasm kills the living and empowers the dead. Once the ectoplasm gets to the killing point, it becomes as strong as it can be, and then creates a ghost. However, if there's another thing killing the person at the same time, then the ectoplasm gets confused.
They both have hit the killing point at the exact same time, and upon doing so, the person is right there in the middle of that point between life and death for a fraction of a millisecond. During that fraction, they flicker back and forth due to whatever is killing them besides the ectoplasmic radiation. This makes the ectoplasm confused because it doesn't know whether this is a living thing that it needs to kill or if it's a dead thing that it needs to empower. However, the default makes it empower the dead before it kills the living. Therefore, it empowers the person being killed, and it wards off whatever else is killing the person. In other words, it saves them from it, thus leaving them in between life and death.
So, technically, they are still alive and the ectoplasm has empowerd them. This makes them not only immune to ectoplasm, but also able to absorb it and utilize it. If they work on utilizing it enough, then they'll get better, as seen in the show.
So, that's my theory on ectoplasm and how halfas are created. I hope you like it. ;)