According to Folklore full upirs are the spawn of witches and demons and a half-upir is 1/2 human, 1/4 witch, and 1/4 demon. It can be assumed that they have been around a long time, perhaps since the beginning of time. Scientifically, the upir gene is identified as a heritable retroviral anomaly. This anomaly in the certain genomes is what turns people into upir, but it can only be genetically inherited. It appears that those that are not full upir but are a portion of it or have strong demonic gene in their bloodline will be prone to develop strange deformities.
After death by suicide, they become full upir, biologically immortal creatures with greatly enhanced strength, accelerated healing capabilities, as well as unnatural jaw flexibility, iron-strong teeth, toxic enyzmes, and a nearly insatiable, rabid appetite for blood and raw tissue. Upirs can become sick or have diseases only on the rarest of occasions, such as having a congenital heart defect, which is fatal to them. Once a human has transformed via suicide into the immortal your, they spend months or even years adapting to and controlling their thirst for blood or raw tissue. The new Upir has almost no self-control over their hunger and is prone to dangerously aggressive behavior. Even as they age and learn to exercise some self-control they will revert to instinct during dangerous or violent situations.
Marinette Dupain-Cheng had this gene though she didnt know it but it wouldnt be long until she found out.