Bacterial infection of the lungs
Viral infection of the lungs
Fungal infection of the lungs
Aspiration of food, liquid, vomit, or saliva into the lungs
Inhalation of irritants or toxic substances
Weakened immune system
Chronic lung disease
Smoking
Advanced age
Recent surgery or hospitalization
Use of a breathing tube or ventilator
Poor cough reflex or difficulty swallowing
Exposure to infected respiratory droplets
Living in crowded or long-term care settings