COVID-19 is a viral disease induced by infection with the novel SARS-CoV-2 coronavirus.Infection arises from the inhalation of viral particles spread by an infected individual.The main virus entry pathways are the nose and mouth mucous membranes, particularly richin ACE2 receptors, that are used by the virus to enter cells and begin its replication.Not all individuals who contract the virus fall ill, and most of those who do (about70-80%) experience mild symptoms (fever, cough, headache, muscle aches, loss of taste andsmell). If the immune system is unable to rapidly fight the virus back, the latter canattack the lungs, affeting oxygen absorption inside the alveoli and lead to pneumonia.This more severe form is observed in 20% of patients and can develop into even moreserious complications requiring intensive care, with a high mortality rate. In severecases (about 5%), the disease spreads to the entire body reaching the cardiovascularsystem and brain, with the risk of heart attacks, encephalitis and stroke, but it canalso affect the liver, kidney and bowel.Reasons why some subjects develop a highly severe disease while others have little or nosymptoms at all still remain unclear. Many researchers are trying to find an answer byinvestigating the human genome, and the Mario Negri laboratories are also working in thisdirection.The general hypothesis underlying this research project is that inter-individual geneticvariations can explain the different responses to viral infection in a population, andthat COVID-19 severity is, therefore, genetically determined in each infected subject.