Some people are generally susceptible to upper respiratory infections (URI's) and they are one of the most common forms of infection around.  URI's include the common cold, allergic rhinitis and rhinosinusitis, pharyngitis, epiglottitis and tracheobronchitis.  This kind of infection can be caused by both viruses and bacteria.  When you have conditions where you have high exposure to respiratory pathogens and airborne toxins, for example from bushfires and smoke drift, along with acute or chronic stress, then you have a recipe for higher and more severe incidents of URI's and aggravated allergies.