The ‘hygiene hypothesis’ was first proposed in 1989 by an epidemiologist, Dr Strachan, he reported an inverse relationship between family size and development of atopic disorders (asthma, atopic dermatitis, hayfever, and food allergy), and proposed that a lower number of infections in early childhood, or acquired pre-natally could be a cause of the rise in allergic diseases .
The concept was further studied by allergists and immunologists, it evolved into the broader concept that declining microbial exposure is a major factor in the increasing incidence of atopy.
A wide range of factors which might have resulted in altered microbial exposure have been examined such as clean water and food, sanitation, antibiotics and vaccines, birth practices, as well as incidental factors such as the move from farm to urban living.
The rapid increase in allergic asthma and other atopic disorders in the industrialized world is usually considered to have started between 1960 and 1970 with progressive rises during the 1980s and ‘90 s, Asthma increased by about 1% a year from around 1980.
Both incidence and prevalence of atopy remain much lower in many, developing countries. The International Study of Asthma and Allergies in Childhood (ISAAC) showed that the prevalence of self-reported asthma ranged from 2–3% in developing countries to 20–40% of the responding population of 13–14-year olds in industrialized countries. 
The reunification of Germany provided some new insights into the influence of lifestyle on atopic disease. Hayfever and atopic sensitization among children in the former East Germany both significantly increased between 1991–1992 and 1995–1996, raising the issue of ‘Western living’ influences on children, as previous studies had shown lower rates in East Germany compared with West Germany. 
1. Hay fever hygiene and household size. Strachan DP. Br Med J. 1989;299:1259–60.
2. Worldwide variation in prevalence of symptoms of asthma, allergic rhinoconjunctivitis, and atopic eczema. Beasley R, Keil U, von Mutius E, Pearce N. : ISAAC. Lancet. 1988;351:1225–32
3. Increasing prevalence of hay fever and atopy among children in Leipzig, East Germany. von Mutius E, Weiland SK, Fritzsch C, Duhme H, Keil U. . Lancet. 1998;351:862–6.