If vampires were real, how did the first vampire became one?

In movies, you turn into a vampire when one bites you and gives you their blood. They basically killed you.

The vampire that bit you became one too when he/she got bitten. But going back to the very first vampire ever, how did she/he became one if there weren't any vampires? Would it be through an animal then? how?
If vampires were real, how did the first vampire became one?
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