This chapter contains sections titled:

• Introduction

• International studies of asthma and other atopic disorders: geographic variation

• Differences in prevalence of allergic diseases in rural and urban communities

• Atopy and allergic diseases are not the same

• Tools used to assess prevalence of atopic disorders and of atopy and their drawbacks in international studies

• Exposure to microorganisms and parasites: risk or protective factor?

• Cellular immunologic profiles associated with allergic disorders in developing countries

• Concluding remarks

• References