Setting: "One Big Lie" Sci-fi RPG
The lies in this setting are an extension of relativity with a couple of extra dimensions allowing for FTL/Backwards Time Travel, and Thaumons.
Thaumons (Magic Bosons) are a class of particle that can be "programmed" by tuning their mass such that they decay in a roughly predetermined way. This is used by Thaumaturgists (Wizards) to power the VIG (Hyper-space-drive) and their Wands.
This can be used to generate substances (blast water or fire), affect quantum fields (zap with lightning), move things (typical telekinesis) and scry (seeing the past or things far away). It cannot be used to shapeshift, send psychic messages, or mind control anything.
Unbeknownst to mankind, another lifeform has been adept at the manipulation of Thaumic Fields for Centuries. A microbe has perfected the telekinetic manipulation of blood that it has already infected. It generates EMF to communicate with the nearby microbes and, when lots of them get together, the EMF they collectively generate starts to look very brain-like. The EMF is also used as a "beacon" for the telehaemokinesis which is why it can't use blood it has not infected.
The microbe has recently found a dead human and crawled its way inside, taking over its entire blood stream, forcing the heart to beat, the lips to form words, the legs and diaphragm to move giving the appearance of life. However, the brain is not active.
Collectively, these creatures are not stupid. They find the idea of human hosts incredibly useful, but the longer they spend in human hosts, the more they learn of morality. They eventually decide that human beings are worth respect.
They still need corpses. They still need fresh blood.
So they live in the bodies of the naturally deceased, as they adjust to their new homes and fully take over all the cells the body appears ill for about a month. They use this time to learn more about the human they inhabit and copy who he was. The month long illness would appear to most to be quite severe. Long bouts of unconsciousness, pale skin, very weak muscles, lots of coughing... the changes in the person afterwards are usually put down to how long they were out of the loop and people adjust to the new man before them as it adjusts to them.
But how do they get blood without revealing themselves or arousing suspicion? If I made a living taking over corpses I'd feel compelled to keep it under wraps.
The blood must be drawn from a "similar" animal. So ideally a primate but if there's no better way any mammal will do.
The blood must be "fresh". If the blood is not alive when the vampire drinks it, then its likely to be uninfectable [sic]. Very recently dead is OK, but every second counts.
Due to their new-found moral concerns and care for humans, they would rather not drink blood from non-consenting humans or steal it from blood banks where it is used to heal the sick.
No human can be left with knowledge of them after feeding.