Bronchitis is mostly caused by the invasion of pathogenic bacteria into the body, affecting the bronchi. Common pathogens include Streptococcus pneumoniae, rhinovirus, adenovirus, mycoplasma, chlamydophila, and it might also be due to infection by pathogenic bacteria such as Bordetella. It often occurs when the body's immune system is compromised, due to staying up late, being exposed to cold, lack of sleep, or getting wet in rain. During the acute phase, symptoms such as high fever, chills, coughing, spitting phlegm, and breathing difficulty may occur. It's important to detect and treat it early to prevent it from turning into chronic bronchitis.