You Qian
"Humans nowadays eat tons of additives, melamine, preservatives, gutter oil, and other god knows what. What would happen if we got food poisoning from drinking human blood?" Vampires have a hard time
"Do you know a species that only eats one kind of food?" Yeah, its called the "panda," perhaps you have heard of it? And, off the top of my head, the koala, the pen tailed treeshrew, that one glider species that also only eats eucalyptus (it's either the greater or lesser glider, I forget which), the anteater, leafcutter ants, all varieties of the cordryceps fungus (each species of fungus will only infect and consume one species of insect), and every single virus that only infects a single species. Biology is weird.