Your immune system sometimes snoozes

Most of the white blood cells patrolling your body right now will be dead by Tuesday. While your red blood cells live a long, stable life of about 120 days, the average white blood cell survives for only 1 to 3 days.

These cells are the disposable infantry of your immune system. They are designed to hunt, engage, and often destroy themselves in the process of neutralizing a threat.

When you see pus from an infection, you are looking at a mass graveyard of millions of white blood cells that died in combat. Your bone marrow has to work at a furious pace to keep up, churning out 100 billion new white blood cells every single day.

Some specialist cells, like B and T memory cells, are the exception. They can live for years or even decades, acting as the 'veterans' that remember how to fight a specific virus you encountered in childhood.

This constant cycle of birth and death means your entire internal defense force is almost entirely new every week.