The purpose of this observational study was to characterize individuals in Mexico who sought medical care for influenza-like illness (ILI) and Severe Acute Respiratory Infection (SARI), describe seasonal pattern of the pathological agents associated with ILI and SARI and to identify changes in severity of disease caused by a specific agent. This study started on January 2015 and was completed on November 2016 with an enrollment of 888 participants.