The prevalence of allergic diseases has significantly increased over the last two decades. Rather than genetic factors, which may not have played a significant role over such a short period of time, factors related to the Western style of life and to the socio-economical level of our society have been incriminated. Indeed, significant differences were demonstrated in terms of sensitization to various allergens, and expression of allergic diseases such as allergic rhinitis, atopic dermatitis or asthma between former East Socialist and Western countries, between highly industrialized and under developed regions. The shaping of the immune system by exposition to infections may play a crucial role very early in life; the absence of exposure in contrast may lead to preferential expression of an atopic phenotype. Other factors may also play a role such as breast feeding, fat consumption, and of course allergen exposure. These epidemiological observations open new questions on the possible need for changes in our style of life.