How can you turn into a vampire?

No, vampires, in the classical form, (most likely) do not exist in the real world, so you cannot become one.

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In another sense, there definitely are certain individuals who can "suck the life out of you," by their demanding, selfish behavior. Sometimes this is perfectly obvious and obnoxious, as when an adult behaves like a spoiled child who never grew up to consider others. But often it can be very subtle. There is such a thing as a "psychic vampire," who makes demands that first seem reasonable, but soon become very toxic and spiritually deadly. There are certainly charismatic persons who attract many followers, only to completely destroy their lives. The Rev. Jim Jones, who led hundreds of innocent, gullible followers to their deaths is one example of such a "vampire."

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The above rude definition of a psychic vampire should not be confused with those persons who need to take psychic energy from other people in order to maintain their own health and energy levels. Real psychic vampires prefer to take energy from willing donors, who do not come to harm from the process.

There are also real sanguinarian vampires, who take small amounts of blood from willing donors.

The party line is that you must be born one, and cannot become one. I prefer the truth, which is that turning is possible for certain types of psychic and sanguinarian vampires, but the detriments outweigh any possible benefits, so why would you ever want to?

(Just for starters, in regards to sang vampirism--nothing in life changes, except that the sun hurts your eyes and is uncomfortable, you feel tired all day, you have a need that will consume your thoughts if you don't feed, and you suffer from various other physical discomforts if you don't feed. That's all--no special powers that you couldn't have learned as a non-vampire, no fangs, no super-strength or speed, and the slightly improved night vision just isn't all that spectacular).

So, again...the question isn't "can you become a vampire", it's 'why would you want to?"