Pneumonia is characterized by lung infection and _____________.

Pneumonia is primarily characterized by an infection in the lungs, which leads to the inflammation of the air sacs (alveoli). When this occurs, the alveoli often fill with fluid or pus, making it difficult for the individual to breathe properly. This collection of pus in the alveoli can contribute to the hallmark symptoms of pneumonia, such as cough, fever, chills, and difficulty breathing. Understanding this condition's mechanism highlights the importance of recognizing pneumonia as an infectious disease in which the alveoli are compromised, leading to reduced oxygen exchange in the lungs.

The other options do not correctly describe a fundamental aspect of pneumonia. Smoking and conditions like asthma or COPD, while they may have implications for respiratory health, do not define pneumonia itself or specifically relate to the infection and its resulting effects on the lungs.