A friend/colleague of mine is suffering from Chronic sinusitis and she usually sneezes every morning for around 20-30 times (random number, I know!) over period of few hours. The 'sneezing fit' is 100% of the time non-contagious yet loud and 'full with watery substance', She covers her sneezing most of the time however it is not always possible to react in time.
Also, she's a single working mother raising two well mannered and cute children. Would you expect her to always stay home when she's always 'sick'?
These kindergarten-level cultural learnings you are talking about are societal pressure, we inhale pollutants, pollens and smoke from cigarettes all the time!