Nora suffers from anemia, "a condition in which you don't have enough healthy red blood cells to carry adequate oxygen to your tissues" (source). Anemia is a relatively common condition, and it isn't usually life-threating, but it can make people feel weak, dizzy, or tired. Nora mentions several times that she has to take iron pills to combat these symptoms.
Not only does this condition make Nora feel physically weak at times, it's also a symbol of other things going on. A lot of times in literature and art, blood represents a person's identity and the forces driving that person's life. Nora has a problem with her blood: It lacks the ability to carry enough oxygen throughout her body. Could that be a sign of lack elsewhere in Nora's life? Is something missing for our main girl?
Moreover, we learn that Nora actually has bad blood running through her veins, and not just in terms of its inability to deliver oxygen—it's bad in that it carries her Nephilim heritage. When Nora has to duke it out with her Nephilim-side relative, Jules, she finds herself in major need of her iron pills:
Blood drained from my head, and I felt myself start to slip off the chair. I'd felt this way enough times before to know I needed iron. Soon. (28.65)
But when you're locked in mortal combat, it's kind of hard to stop to go seek iron. Nora feels these same affects—dizziness and wobbliness—as she climbs the ladder and rafters to escape Jules, though no doubt the sensation of being high in the air and needing to balance on a thin beam also contribute to her feelings. At this point, Nora hasn't taken anything to help with the low-iron episode. However, she is able to speak, think, and act with confidence, and she defeats her Nephilim kinsman.
Readers who have experience with anemia have pointed out that symptoms are not things you can willpower away, so it seems the book may be working with the condition more on the symbolic level rather than on the practical level (source). Is Nora's ability to overcome the symptoms of her anemia representative of her ability to destroy her evil blood relative and reject the bad blood running through her family's history? Do you think she will have to face the implications of her bad blood again in the next book of the series? Are you singing Taylor Swift's "Bad Blood" in your head right now?