Most cases of asthma are due to allergies or hypersensitivity to some environmental factor. Sometimes this hypersensitivity does not fall within the medical definition of an allergy and is referred to as 'intolerance', 'sensitivity' or 'idiosyncratic reaction'. To the sufferer these terms mean very little. He or she reacts by experiencing breathing difficulties when exposed to a food, a chemical or an inhaled particle, in a way that other people do not. It is quite possible to become suddenly sensitive to something that has never bothered you before, as a result of a viral infection or a sudden exposure to a strong chemical. This is sometimes called a pseudo-allergy and the allergic reaction often disappears once the underlying problem is successfully treated. But sometimes the reaction/allergy lingers on.
Asthmatic people may not even know that they are allergic, but the very nature of the condition makes it clear that they are allergic or over-reacting to at least one (but usually several) factor in their environment. Environmental factors include anything a person touches, breathes, drinks or eats. It may be pollens, moulds, fungi, a common food (such as milk) or a natural chemical contained in foods. Often it is a toxic, synthetic chemical. These are known as 'xenobiotics' (foreign chemicals).
Sometimes an allergy or hypersensitivity is masked and only becomes apparent when a period of avoidance is followed by renewed exposure. In general one should bear in mind that, although the frequency and severity of symptoms can vary greatly, one can not be a little allergic or asthmatic any more than one can be a little pregnant. Many women sail through nine months of pregnancy without a single complaint and have a moderately distended abdomen at the fifth month. Others have distended abdomens by the third month and more still are plagued with morning sickness, suffer high blood pressure or eclampsia. The women with more distended abdomens and those unfortunates who suffer morning sickness are not more pregnant than their less obviously pregnant friends and those who experience no discomfort.
In the same way, the severity of symptoms in asthma is not an indication of the severity of the condition. In fact, statistics show that a disproportionate number of deaths from asthma occurs among those who are regarded or regard themselves as 'mild asthmatics'.