Hayfever is a very common condition, affecting about 500 million known sufferers every year. It is caused by an allergy, often to pollen and sometimes mould spores. In hayfever the body's immune system over-reacts to the presence of external substances, as if they were something foreign. This results in irritation and inflammation.

Hayfever often runs in families, and is also related to asthma and eczema. It is quite common to find a family with some members with asthma, some with hayfever, and some with eczema. Any individual might have more than one of these conditions.

Different pollens and allergens are present at different times of the year and thus the time that you are affected depends on the trigger to which you are allergic. Other factors which may aggravate the situation are the weather and the air quality.