What causes a persistent cold flu?

Resistance. Everyone gets colds and flu but symptoms can persist if resistance is low from fatigue , allergies, vitamin deficiency or chronic illnes.
Persistant Cold/Flu. When a "cold/flu" last longer than ten days, it no longer is a cold/flu -- it has moved on to something new. Examples are allergies, sinus infections, chronic acid reflux, or anatomic blockage from many sources. When a cold last longer than ten days, suspect allergies or a sinus infection. See your doctor to decide which one.