Pneumonia may be caused by viruses, bacteria, or fungi. Influenza and respiratory syncytial virus are two common causes of viral pneumonia (RSV). Streptococcus pneumoniae is a common cause of bacterial pneumonia (pneumococcus). Pneumocystis pneumonia (PCP) is caused by a fungus (Pneumocystis jirovecii) and can affect people who are immunocompromised, such as those who have HIV/AIDS. Pneumonia may also be caused by using a ventilator, a condition known as ventilator-associated pneumonia (VAP). Pneumonia's pathophysiology follows a consistent pattern. When pathogens infect the lungs, the inflammatory response causes fluid to build up in the alveoli. Hepatization occurs as capillaries dilate and neutrophils, red blood cells (RBCs), and fibrin fill the alveoli, giving the lung a red and granular appearance. Following that, blood flow decreases, causing leukocytes (white blood cells) and fibrin to invade and consolidate in the region (solidify). The consolidated substance dissolves as the infection clears, and macrophages absorb and extract it.