respiratory tract infection

Lower – respiratory – tract infections affect the trachea, bronchi, lungs, & alveoli.
Problems associated with the Lower – respiratory – tract include respiratory distress due to bronchitis, and pneumonia (whereas the pathogen acquired route by the oropharynx / nasopharynx – respiratory droplets). The lung tissue provides a moist environment for the pathogen to multiply and infect nearby lung tissues → inflammatory response mediated → dilation of bronchioles → lung impairment (rales may be heard & supplemental oxygen required)

An example is TB, Mycobacterium tuberculi is resistant causing productive cough & chest pain.
 
On the other hand, associated Upper– respiratory – tract problems are: common cold / viral rhinitis, local reaction such as allergic rhinitis, sinusitis (narrowed passages that cause accumulation of secretion promoting bacterial growth), Strep, or laryngitis.

Therefore, Lower – respiratory – tract infections are more fatal because, all the defense mechanisms are in the upper – respiratory – tract!