Vampirism is a spiritual disorder in which a person's spiritual body cannot produce or otherwise supply enough energy for the person to thrive. They do not typically get damaged, just have extremely poor health, when cut off from their feeding source.
It is neither a virus nor genetic. It seems to be something you are born with, though most people report "awakening" to it near puberty. Vampirism itself is caused by not being able to produce enough spiritual energy to thrive upon (explanations for what that happens varies). This low energy makes psychic vampires very sensitive to energy and will generally experience fatigue and pain from lack of feeding or from energy sources too harsh for them.
The word 'vampire' is given to such people who are basically suffering from energetic disorder in a metaphorical value. This basically gives us a better understanding of the term by giving us a simple metaphor which we can all easily understand. People have to use their minds, that's all.
To say it very shortly and not make these long, these are people who suffer from a spiritual disorder. That's all.