Metapneumovirus may be the second most common cause of lower respiratory infection in young children (after the respiratory syncytial virus). Studies have shown that by the age of five, virtually all children have been exposed to the virus and re-infections appear to be common. Human metapneumovirus may cause mild respiratory tract infection. However, small children, elderly and immunocompromised individuals are at risk of severe disease and hospitalization. While there are geographical differences in seasonality and incidence of MPV infection, this virus undoubtedly plays a significant role in respiratory illnesses in the pediatric population. Limited research has been done to determine the incidence of MPV in adult populations even though MPV infection has been well established in high-risk adult populations. These include those with chronic obstructive pulmonary disease (COPD), elderly patients, and immunocompromised patients. MPV has been documented as a significant cause of illness in transplant recipients. Studies have linked MPV with idiopathic pneumonia, fulminant respiratory failure, and high mortality rates in stem cell transplant recipients. Additionally, in one study, MPV was found in 10% of lung transplant recipients with acute respiratory tract infections, similar to the rate of RSV detection. Transplant patients appear to be at significant risk for severe MPV illness.