Asthma prevalence in New Zealand is amongst the highest in the world and there is no cure or effective prevention. In a previous study in 50,000 infants followed-up for 18 years, we have shown that biodiverse green space was associated with a reduced risk of asthma. We hypothesise that this may be due to increased environmental and human microbial diversity, which have been shown to direct the immune system away from developing allergies and asthma. The proposed study in 900 urban children from Wellington will assess, for the first time, whether biodiversity reduces allergy and asthma risks through differences in indoor and human commensal microbiota and specific immune responses, whilst adjusting for effects of nutrition, physical activity and stress. The overall objective is to increase our understanding of modifiable causes and mechanisms of allergies and asthma, enabling the development of novel and targeted interventions.