The easiest way to put it is that he was born that way.

[ He knows this is confusing, and after that shock... He knows he needs to elaborate more, perhaps explain everything. There are things he'd rather keep to himself, and that already makes the conversation much more difficult. However... Sano has already proven to be a trustworthy man, one who will at least not attempt to bring harm unwarranted. ]

He is a vampire, but unlike me he can tolerate sunlight and running water perfectly, as any human would. For the time being, he also doesn't need blood and can sustain himself on food. The trade-off is that, currently, he is powerless.