Emerging evidence indicates that COVID-19, which is the disease that SARS-CoV-2 causes, leads to blood clots in an estimated 30% of critically ill patients. A blood clot, or a thrombus, increases the risk of complications and death among those who have COVID-19. This disease causes blood clots in 20–30% of critically ill patients. One theory is that the clotting occurs when the novel coronavirus attacks the endothelial cells that line the blood vessels. The virus does this by binding to the ACE2 receptors, which are present in the endothelial cell membrane. Once the virus binds to these receptors, the blood vessels start releasing proteins that trigger blood clotting. Another theory is that COVID-19 causes the body’s immune system to trigger a hyperactive inflammatory response. This inflammation may then trigger clotting. Other factors may also play a role in blood clotting in people with COVID-19. For example, many people who require hospital care due to the disease also have other risk factors for blood clots. Experts do not fully understand why the novel coronavirus causes blood clots in some people. These clots typically develop in the lungs, but they may also develop in other areas of the body. Blood clots increase the risk of complications, including stroke, heart problems, and death. People who have concerns about their risk of blood clots should speak to their doctor. However, it is important to remember that most people who develop COVID-19 will develop mild-to-moderate symptoms and recover without complications.