All living cells require iron to live, this includes bacteria and human cells, because iron has essential functions in the cellular metabolism. The problem is, that free iron is actually toxic, so that each organism needs to make sure iron is bound by a special vehicle.
In the human body there are many different iron-carrying vehicles. Most of the iron in our body is stored in a special molecule which is called haem. It looks like the ring structure in the figure below with the iron (Fe) in the middle.
Haem is part of a protein called haemoglobin which is located inside our red blood cells. The iron inside the haem inside the haemoglobin is the one that binds oxygen and carries it to all our cells.
Interestingly, when haem is bound to oxygen, it turns red, which is the reason why our blood is red. Without the oxygen, the haem complex is blue-red.
Anyway, most of our iron inside the body is stored within these haem molecules. And many pathogenic bacteria developed strategies to survive within the body by stealing our haem and thus our iron.
If you want to find out more about this strategy, read here:
about the research article here: