A person with a secondary infection is a patient who has not been cured of the primary infection and has developed an infection from another pathogen. Secondary infections, also known as post-primary infections, are caused by re-infection with other pathogens, such as bacteria, fungi, and viruses, during the course of an illness. Some primary illnesses reduce the immune system’s ability to fight off harmful invaders such as viruses and bacteria, leading to secondary infections. Common examples are patients with secondary vaginal yeast infections after taking antibiotics to treat infections caused by bacteria; and patients with upper respiratory tract infections (e.g., colds) caused by viruses followed by pneumonia caused by bacteria or fungi. Secondary infections need to be taken seriously, and the pathogen should be identified as soon as possible to avoid aggravation of the condition.