viruses, Tiny particles that cause colds, immune system, The body system that fights sickness, symptom, A sign of illness, infect, To make someone sick, reproduce, To make more of the same kind, sneeze, A common cold symptom involving air from the nose, cough, A body action that helps remove mucus, evolve, To change over time, spreading, The action of moving sickness from one person to another.

A Common Problem

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