Some food allergies can be fatal. Another common source of allergy is dust and pollen. Modern living seems to produce more dust and therefore more dust mites, horrible little creatures, which live in the dust. They love thick carpets, double-glazing and central heating. So - remember, open the windows and vacuum frequently.
People can be also allergic to drugs - penicillin, for example.
Some of us are allergic to synthetic products, and some women are allergic to certain types of cosmetics. SAD is a form of clinical depression that strikes many people during the short daylight hours of winter. During the darkest months of the winter melatonin production increases. Because of the lack of light. SAD is especially common in northern latitudes, where winter daylight is much shorter.