Orbs often seen in ghost hunting aren't really ghosts but simply energy moving around, collective balls of energy.
Fairies are often depicted as little balls of light often in which an entity is seen drawn as a little winged femme or whatever. Faes being orbs that have collected some level of conciousness or subconciousness, capable of having an agenda and simple thought expressed through emotions.
Sprits or ghosts were human, and if you believe in reincarnation, become human again.
Life stage to make a Spirit: Orb, Fae, Human Spirit. Or other animal. Maybe it depends on the amount of conciousness the fae has before evolving and then becoming born. The idea that a spirit enteres a couple during (or after) sex and gives conciousness to a zygote to create a new being (human, cat, etc depending on the couple's species).