what causes Bella to faint

While human, the scent of blood.
She sometimes faint in the presence of Edward! LOL. I honestly can't understand that.

the smell of blood
which she says smells like rust and salt

On Google and wikipedia, it says: Allergic reactions to blood are possibe, but in Twighltight, it is just the smell.

Blood in school while her class was taking samples for donations

Almost the Edward'kisses~

while human blood makes her faint coz she can smell it n she doesnt lyk it at all