Introduction

Chronic inflammatory diseases (CIDs), such as allergic and autoimmune diseases, include a wide array of diseases in which the immune system is thought to cause disease through excessive or dysregulated immune reactions. Since these diseases are only partly genetic[1], many environmental factors have been examined[2,3]. Environmental factors implicated include occupational/environmental chemical exposures[4], diet[3], infections[5–7] and stress[8,9].
Over the last 100 years, many of these CIDs have been increasing in Western countries, and recently have increased in association with westernization in developing countries[3,10,11] The term westernization, as used here, refers to the adoption of practices associated with Western culture that potentially impact health. The westernized diet typically includes increased consumption of animal products, fat (especially animal-derived fat and vegetable oils), sugar, ultra-processed foods/beverages, salt, and food additives. New exposures from xenobiotics/pollutants and pharmaceuticals are also included.
The hygiene hypothesis is one of the hypotheses proposed to explain the westernization-associated rise in chronic inflammatory disease[12]. The hygiene hypothesis originally attributed the allergic disease increase to decreasing rates of infections. These infections were proposed to “train” the immune system to not react to harmless allergens. Updates to the hygiene hypothesis have been proposed that focus on the reduction of microbes that humans were exposed to throughout evolutionary history[13–15]. These updates to the hygiene hypothesis will be referred to here as the altered microbiota hypothesis. In general, they propose that reduction in likely coevolved, often commensal, microbes lead to microbial and immune system imbalances that can give rise to chronic inflammatory diseases.