In physics, when you magnetize a material in one direction, such as by holding a magnet to another piece of metal, the metal does not fully demagnetize after you remove the magnet. In biology, the T cells that help power your immune system, once activated, thereafter require a lower threshold to reactivate. Hysteresis describes how both the metal and the T cells partially remember their states, such that what happened previously can impact what will happen next.
