When pathogens enter your lungs they start to multiply and trigger an inflammatory response, causing the air sacs to fill with fluid. During the first 24 hours, fluid buildup leads to heavy congestion in the lungs. This results in coughing and extreme fatigue. Red blood cells and immune cells gather around the alveoli, leading to inflammation and reduced oxygen transport. Fibrous tissue increases as red blood cells are destroyed, giving the lungs a grayish appearance. As the infection progresses, the lungs' ability to exchange oxygen is impaired, making it difficult to breathe. As your immune system fights the infection, it also contributes to the inflammation. Eventually, the inflammatory fluids and damaged cells clear, allowing the lungs to recover and return to normal function.