Respiratory disease is a major problem in pig production worldwide causing mortalities and impaired growth with reduced welfare and economic losses. There is increasing evidence that respiratory disease is not always linked to single pathogens but multifactorial, referred to as the porcine respiratory disease complex (PRDC). PRDC can involve well-known pathogens and perhaps others, previously undefined ones. However, exactly which pathogens are involved, how they interact and affect severity of disease is poorly understood. This hampers effective management, treatment and ultimately control of respiratory disease in pigs. This project aims to explore which bacteria and viruses are involved in PRDC, how pathogens interact, if they vary over time, and how environmental and management factors contribute to respiratory disease. This will provide practitioners with updated knowledge enabling tailormade interventions at herd level to reduce the impact of respiratory disease.