Human coronavirus NL63 (HCoV-NL63) is a species of coronavirus, specifically a Setracovirus from among the Alphacoronavirus genus. It was identified in late 2004 in patients in the Netherlands. The virus is an enveloped, positive-sense, single-stranded RNA virus which enters its host cell by binding to ACE2. Infection with the virus has been confirmed worldwide, and has an association with many common symptoms and diseases. Associated diseases include mild to moderate upper respiratory tract infections, severe lower respiratory tract infection, croup and bronchiolitis. The virus is found primarily in young children, the elderly, and immunocompromised patients with acute respiratory illness. It also has a seasonal association in temperate climates. A study performed in Amsterdam estimated the presence of HCoV-NL63 in approximately 4.7% of common respiratory illnesses. The natural reservoirs are palm civets and bats. Estimates of its divergence from another coronavirus (HCoV-229E) are around 1000 years ago; it has likely circulated in humans for centuries. The evolution of HCoV-NL63 appears to have involved recombination between an ancestral NL63-like virus circulating in African Triaenops afer bats and a CoV 229E-like virus circulating in Hipposideros bats. Recombinant viruses can arise when two viral genomes are present in the same host cell. Symptoms The first cases of the infection with HCoV-NL63 were found in young children with severe lower respiratory tract infections admitted to hospitals. While the clinical presentation of the virus can be severe, it has also been found in mild cases of respiratory infection. The comorbidity of HCoV-NL63 with other respiratory infections, has made the specific symptoms of the virus difficult to pinpoint. A study of clinical symptoms in HCoV-NL63 patients without secondary infection, reported the most common symptoms to be fever, cough, rhinitis, sore throat, hoarseness, bronchitis, bronchiolitis, pneumonia, and croup. An early study investigating children with lower respiratory tract illness, found that HCoV-NL63 was more commonly found in outpatients than hospitalized patients, suggesting that it is a common cold virus similar to HCoV-229E and HCoV-OC43, which generally cause less severe symptoms. However, the high frequency of croup is specific to HCoV-NL63 infection. Cause It is believed that the route of HCoV-NL63 spread is through direct person-to-person transmission in highly populated areas. The virus can survive for up to a week outside of the body in aqueous solutions at room temperature and three hours on dry surfaces. Most people will be infected with a coronavirus in their lifetime, but some populations