Over 35 million people have donated their marrow worldwide, according to the World Marrow Donor Day, which took place September 21. That's 35,295,060 who've selflessly given a part of themselves so another person can have a shot at life. World Marrow Donor Day celebrates and thanks those millions of people who have donated cells for blood stem cells or marrow transplants. But how do you really say thank you to someone who saved your life?

Eighteen-year-old Jack Santos wasn't aware that he was sick."I was getting a lot of nosebleeds but I didn't really think I felt anything wrong," Jack told ABC news. During his yearly checkup, his bloodwork revealed that he had aplastic anemia, a rare non-cancerous blood disease in which there are not enough stem cells in the bone marrow for it to make new blood cells. There are 300 to 900 new cases of aplastic anemia in America each year. It is believed that aplastic anemia is an auto-immune disorder, but in 75% of cases, the cause of the disease is unknown.

It wasn't easy for his family to see him struggle with the illness. "I didn't want to see him go through something like this," Shelby, his older sister, said. "It was terrifying, but we were ready for whatever brought with it at the time."