During the 1918 Spanish flu pandemic, doctors discovered they could treat sick patients with the blood of those who had already recovered. The therapy, known as convalescent plasma, helped reduce mortality among people with acute infections. Now, the FDA has issued an emergency authorization for the therapy's use among COVID-19 patients.

Antibodies develop in plasma, the liquid portion of blood — they're part of our body's natural response to a foreign pathogen. So the idea behind the treatment is to help sick people mount an antibody response to the virus by transferring plasma intravenously from those who already have antibodies. 

"What we really need are drugs that, when given early, can prevent a symptomatic person from requiring hospitalization or very dramatically diminish the time that they're symptomatic," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told Facebook founder Mark Zuckerberg last month. 

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