How does a respiratory tract infection result in respiratory failure?

Exhaustion: A respiratory tract infection can increase work of breathing due to inflammation of the airways or filling of the airspaces with fluid; this increase in work can go beyond what the patient can sustain, especially if they are older and otherwise sick.

Worsening: Respiratory infection can become pneumonia, increase in size, and cause much of the lung to be not functional. There is also necrosis of the lung or death of the tissue causing a nonfunctional lung tissue. It can also lead to sepsis which can overwhelm the person and cause multi organ failure.