There are various medical disorders and blood diseases that can have symptoms that resemble vampirism.
Anemia is a blood disease in which the red-cell count is low. Symptoms typically include a pale complexion, fatigue, fainting spells, shortness of breath, and digestive disorders.
Catalepsy is a rare nervous system disorder that causes suspended animation with a loss of voluntary movement, rigid muscles, and a decreased sensitivity to pain and heat. A person who is cataleptic can see and hear but cannot move; their breathing, pulse, and other regulatory functions are drastically slowed, where the person looks dead to the untrained eye. In the past, it was not uncommon for cataleptic people to be declared dead.
Photosensitivity can be caused by certain medications or it can be an inherited gene. The disorder can cause photoallergic dermatitis (reactions caused by sunlight or other radiant energy sources), phototoxic dermatitis (resulting in hyperpigmentation of light-exposed areas), sunburn, hydroa vacciniforme (blistering), xerderma pigmentosis (or Kaposi disease).
Porphyria is an inherited blood/liver disease that causes 50% normal enzyme activity. Historically, it was thought that by ingesting someone else’s blood, it would replenish one’s own supply, but modern studies have proven this false. Symptoms may include an extreme sensitivity to light, sores and scars that break open and won’t heal properly, excessive hair growth, and tightening skin around the lips and gums.