What is the definition of pneumonia?

Pneumonia is defined as an inflammation of the lung tissue typically caused by an infection. This inflammation can affect both the alveoli, which are the small air sacs in the lungs responsible for gas exchange, and other parts of the lung structure. The infection can be due to a variety of pathogens, including bacteria, viruses, or fungi, which lead to the accumulation of fluid or pus in the lungs, resulting in symptoms such as cough, fever, chest pain, and difficulty breathing. The focus on inflammation specifically highlights the body's immune response to an infectious agent, marking pneumonia as a critical respiratory condition that necessitates prompt medical attention to prevent complications.

Other choices represent different respiratory conditions. For example, excessive mucus production is characteristic of diseases like cystic fibrosis or chronic bronchitis rather than pneumonia. Asthma involves airway obstruction but is not classified as pneumonia since it does not typically involve lung tissue inflammation primarily due to infection. The presence of air in the pleural cavity refers to pneumothorax, which is unrelated to the inflammatory processes associated with pneumonia. Therefore, specifying pneumonia as an inflammation of lung tissue due to infection accurately captures the essence of the condition.