As HP is believed to be an adult disease, children are often diagnosed at the chronic stage of the disease resulting of a long-term exposure to low levels of inhaled antigens. Children can develop subtle interstitial inflammatory reactions in the lung without noticeable symptoms for months [97] . Clinical features in the classic form include non productive cough, dyspnea, malaise, asthenia and occasional cyanosis [95] . Lung function abnormalities are not specific and appear similar to changes observed in other ILD. HRCT abnormalities vary from ground glass attenuation predominantly in the mid-upper zone to nodular opacities with signs of Figure 3 Search for ILD etiology in children. ILD is defined by the presence of diffuse infiltrates on chest radiographs or chest high resolution computed tomography, and abnormal pulmonary function tests with evidence of a restrictive ventilatory defect (in older children) and/or impaired gas exchange. The search for etiology requires a systematic step-by-step diagnostic strategy for identifying: exposure-related ILD; systemic disease-associated ILD; alveolar structure disorder-associated ILD; and ILD specific to infancy.