The first patient in the United States with a confirmed case of the new coronavirus was given an infusion of an experimental drug as a part of his treatment at a hospital in Washington State. One day after taking the drug, the patient started to get better. Four days later, his fever was gone.

A single case study isn’t enough to prove anything. It’s not clear if the medication, called remdesivir, actually helped the patient, or if his improvement was a coincidence. But it’s one of a few drugs, including a combination of anti-HIV drugs, that doctors think might help patients with the new coronavirus.

Remdesivir was developed by the pharmaceutical company Gilead as a treatment for Ebola. It’s a broad-spectrum antiviral drug, and it blocks the activity of a protein that helps coronaviruses make copies of themselves. Research groups identified the drug as a potential candidate for the treatment of coronaviruses in the aftermath of the 2012 MERS outbreak, when another new coronavirus spread through the Middle East. In cell models, it blocks the activity of MERS, SARS (a 2002 coronavirus), and other coronaviruses that are found in bats.

First US #nCoV2019 patient treated with #remdesivir. Hard to wrap your mind around the fact that your pipetting may have helped save a human life #coronavirus #antivirals https://t.co/xHN3Ri6Aag — Timothy Sheahan (@timothysheahan) January 31, 2020

Tests on the new coronavirus show that remdesivir blocks its activity, too, at least in the lab. That result, coupled with the positive outcomes in the Washington patient, were enough for Gilead to launch a larger clinical trial of the drug in new coronavirus patients. The company will test it in a group of 270 patients at China-Japan Friendship Hospital in Beijing — one group will be given the drug, and one group will be given a placebo.

The drug isn’t approved by the Food and Drug Administration or by any other regulatory body. However, it already went through safety testing during the Ebola outbreak in 2014 and 2015. That’s why Gilead is able to test it in sick patients immediately.

“The drug has already been used in humans for Ebola. It’s safe, even though it didn’t do much,” says Florian Krammer, a professor and vaccine development expert at the Icahn School of Medicine at Mount Sinai. The Ebola epidemic also illuminated a new road map for drug testing during an outbreak.

Another clinical trial, of a combination of two anti-HIV drugs, is also underway in a Chinese hospital. Chinese experts recommended that the two drugs, lopinavir and ritonavir, be given to coronavirus patients. They’ve also helped a handful of coronavirus patients in Thailand — including a 70-year-old woman, according to Thai doctors.

“This is not the cure, but the patient’s condition has vastly improved. From testing positive for 10 days under our care, after applying this combination of medicine the test result became negative within 48 hours,” Dr. Kriangsak Atipornwanich, a lung specialist at Rajavithi Hospital in Bangkok, told Reuters.

The combination of lopinavir and ritonavir appeared effective in a handful of patients with SARS, back in 2002 and 2003, and blocked the MERS virus in animal studies.

The importance of those previous studies was emphasized by Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, in a press conference last week. Researchers aren’t starting from zero because work over the past two decades tested drugs in cells, animals, and individual patients. Now, researchers who have devoted their professional lives to this work face new urgency around the most important question: if the drugs actually work in people. “There’s no proven therapy for coronavirus infections,” Fauci said.

The results from the remdesivir trial aren’t expected until the end of April, and it could turn out that none of the drugs under investigation are effective in treating the new virus. But that there are options available — and available so quickly — is a testament to all the research that’s been done before.