Doctors are finding that some patients infected with the novel coronavirus have a propensity towards developing blood clots, which can be life threatening if the clot travels to the heart or lungs. "The number of clotting problems I'm seeing in the ICU, all related to Covid-19, is unprecedented," Dr. Jeffrey Laurence, a hematologist at Weill Cornell Medicine in New York City, wrote in an email to CNN. There are three major reasons why Covid-19 patients might have an especially high risk of clotting. One is that vast majority of patients who become severely ill with coronavirus have underlying medical problems, such as diabetes, heart disease and high blood pressure. These patients -- whether they have coronavirus or not - have a higher tendency to clot than healthy patients. Second, one way coronavirus can kill patients is through a "cytokine storm," where the body's own immune response turns on itself. Patients experiencing that storm, because of coronavirus, influenza, or any other reason are at a higher risk for clotting. The third reason is that there could be something about the novel coronavirus itself that's causing clots. Doctors say it's hard to know exactly what's behind what they're seeing with Covid-19 patients in the ICU.