Hartung recently got the chance to surprise him and share in his incredible recovery.
It was a day the Macias family feared would never come.
Daniel and his wife Maritza were both diagnosed with COVID-19 in late March, but Daniel's condition quickly worsened.
"The doctors told us my dad had a 20% chance of living," said Giselle Aguirre.
Rushed to Pomona Valley Hospital, he was immediately put on a ventilator.
"We knew that he would fight with everything in him, because how much he loved us," Aguirre said.
Feeling like the hospital had exhausted every option, the family learned about an experimental treatment for COVID-19. A recent study from the Mayo Clinic has found transfusions of recovered patients' blood plasma into sick patients is safe.
"Just as this family prayed for a match, I donated, not knowing who my plasma would go to," Hartung said.
"With the help of the American Red Cross, we were able to track my donation right to Daniel," Hartung said.
The Macias family invited Hartung to the surprise of a lifetime.
"This is the greatest honor of my life to be able to do this for you," Hartung said.
Daniel and Hartung, who have now both tested negative for coronavirus, embraced with hospital staff nearby.
"It was just right away the next day, he started doing better and three days later, we took the tube, your breathing tube away, you could breathe on your own only three days later. And then, you know, just you just kept doing better and better and better. And here you are," said Lisa, a registered nurse.
"She's got another dad. He'll be watching over you. You come over. You visit. This is your home," Daniel told Hartung.
COVID-19 isn't just a respiratory disease. It hits the whole body