This comprehensive global review highlights the importance of adequate water, sanitation and hygiene (WASH) for human health. Many diseases are caused by pathogens that are ingested with drinking-water, which circulate due to improper treatment and disposal of excreta and which are propagated by inadequate handwashing and lack of hygiene facilities. Diarrhoeal diseases and respiratory infections are the main diseases that follow these pathways. Inadequate management of water resources, such as ponds, standing water in waste or containers and poorly maintained drains, causes diseases that are propagated by vectors such as mosquitoes. Young children are the most vulnerable to inadequate WASH, which is responsible for 13% of all deaths in children under 5 years. This high disease burden could be largely prevented with existing interventions and prevention strategies, which are described in this report.