Answer
The microorganisms that cause diphtheria and pneumonia are frequently found in the upper pharynx of healthy individuals and relatively few individuals contract these two illnesses because the upper respiratory system is lined with mucous containing anti-microbial chemicals like defensins, lactoferrin and lysozyme. These act against the the pathogens in different ways.
Defensins- acts against gram-positive and gram-negative bacteria, fungi and some viruses.
Lactoferrin- withholds iron so that the pathogens are deprived of an essential nutrient to grow.
Lysozyme- breaks the bacterial cell wall.
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