So we're talking specifically about post viral syndrome. This is a great question - the condition is often poorly understood and always underestimated, and as you highlighted, always frustrating. Sometimes more so than the initial disease.

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The initial viral replication from a mild viral infection triggers a local inflammatory response - hence the nasal drip, swelling, chest congestion, maybe mild sore throat or sinus congestion, etc. A person with full immunity (no underlying immune, sinus, pulmonary disease) will clear a mild viral infection by their immune system in 3-4 days.

Day 3-5 is when sloughing of the superficial mucosal layers peaks (hence day 4 is usually the worst of the viral symptoms), and the exposed underlying tissue has 'increased sensitivity' to dry air and nasal drip.

Once new tissue is in place, the symptoms are fully resolved, but this can take 3-4 weeks and even longer if the cough and nasal dripping persist even longer -- this is why the humidity and decongestants and hydration are important. Conservative therapy (humidifier, vicks on feet and neck, day and night decongestants/'cough syrups', honey, tea, soups, lemon) are crucial to healing - take your grandma's advice.