Interestingly, hay fever was relatively unknown until the onset of industrialisation in the nineteenth century, and today cases of hay fever among people who live in the country and who have been close to nature all their lives is very rare. This seems to suggest that city life and all it entails is a major contributing factor to allergies. Some have also linked allergies such as hay fever with emotional stress, which could perhaps be another reason for its increased pervasiveness in cities.
As a sufferer of hay fever I can vouch for its unpleasantness. Symptoms include sneezing, catarrh, itching and watering eyes, sore throat and lowered immune efficiency, and these flare up at different times of year depending on which type of pollen the sufferer is allergic to. Grass pollens and spores tend to affect people later in the year whereas flower pollens cause the most suffering in the summer months.
For some reason, the immune system of an allergy sufferer rejects allergens such as pollen and sends hoardes of antibodies to combat the perceived intruder. The antibodies then release histamines which cause the typical reactions mentioned above.
There is no known cure, and suppressants such as antihistamines are the usual course for those with hay fever. Some recommend eating local honey during hay fever season, the idea being to get your system used to local pollens and so reduce its reactions against them, but this has been shown to produce very little in the way of actual results (it never worked for me either). Also, it doesn't help when your older brother yells suggestions such as "shut up" and "stop it" when a sneezing fit begins, then beats you senseless when you don't, but this hasn't happened for some years.