Perihilar pneumonitis is when a person gets pneumonia in the perihilar area around the lungs. The perihilar area surrounds the hilium which is where the arteries, nerves, and ducts enter and leave the lung. Pneumonia is a bacterial infection that causes an inflammation in the lungs. The illness can be very severe if left untreated, and many types of pneumonia have become resistant to antibiotics. People who are suffering from perihilar pneumonitis have a fever, deep cough, fatigue, and often mild to severe chest pains.