Bone infection

Besieged Castle with Crumbling Walls

Picture a medieval castle (the bone) with soldiers patrolling the ramparts day and night (white blood cells in the blood vessels). An enemy force (bacteria) breaches the outer wall through a crack from a siege weapon (an open fracture or surgical incision), or they sneak in through the supply line (the bloodstream from a distant infection). Once inside, they multiply in the courtyard (the bone marrow), and the castle commander (the immune system) sends reinforcements—but the castle walls are rigid stone (the cortex), so when thousands of soldiers and supply carts jam in, pressure builds and chokes off the drawbridge (the blood vessels get compressed). Whole sections of wall die from starvation (sequestra form), and the enemy digs tunnels under the outer wall to escape (sinus tracts drain pus to the skin). The only way to win is to tear down the dead wall sections (debridement surgery) so fresh supplies can flow again and the remaining soldiers can reach every corner (antibiotics penetrate living tissue).