Persistent loose mucus after a past lung infection is often due to residual airway inflammation or post-nasal drip rather than active infection, and it can occur with conditions like bronchitis or sinus-related drainage. Drink plenty of warm fluids, do steam inhalation, and a mucolytic syrup (to thin mucus) or antihistamine may help, but avoid taking antibiotics without need. If it lasts longer, worsens, or is associated with cough, fever, or breathlessness, consult a doctor for evaluation and proper treatment.