There are many causes of upper respiratory tract infections, the most common being due to factors such as exposure to cold, staying up late, fatigue, mental stress, and lack of sleep, which lead to a decrease in the body's resistance. The normal pathogenic bacteria in the body gain the upper hand, easily causing respiratory infections. Additionally, it's also possible for patients with weak constitutions to come into contact with pathogens, for example, being around patients with pneumonia or influenza. The exhaled gases from these patients can contain certain pathogens, making it easy to inhale them through the respiratory tract and cause infections.