The Physician's Cure is a potion, invented by the healing god Asclepius, that can bring the dead back to life if administered quickly.
Upon hearing the death of her hunting companion, Hippolytos, the son of Theseus who killed him, a grief-stricken Artemis gathered his deceased body and presented it to her nephew Asclepius, a son of Apollo and the best physician in all of Greece. She asked to have Hippolytos brought back to life, and Asclepius complied by creating the physician's cure and administering it to the dead prince. It revived Hippolytos immediately, but upsetted the natural order of things, causing both Aphrodite and Hades to complain to Zeus about Asclepius' digression. The king of the gods struck Asclepius dead with lightning, angering Apollo who avenged his son by killing the Cyclopes that forged Zeus' Master Bolt. This act resulted in Apollo being stripped of his godly powers for a year, and to prevent further feuds with his son, Zeus resurrected Asclepius and made him into a god. However, the physician's cure and its abilities was declared off-limits medicine. 
When Leo Valdez, Percy Jackson, Hazel Levesque, and Frank Zhang capture the goddess Nike in Olympia, she warns that one of the four present will die battling Gaea. She also tells them of the physician's cure and the necessary ingredients but says it is too dangerous. Frank replies that he likes cheating death and that they will proceed to find the ingredients.
In Pylos, Piper McLean and Frank go to find Frank's ancestors, who might have insight on the Pylosian mint. One of Frank's ancestors gives him the mint, no problem, but warns that their next test will be even tougher, and directs them to Sparta.
In Sparta, Piper and Annabeth Chase descend into some ruins, which is the temple of fear, or Phobos, her maternal half-brother. Guarding the chained god's heartbeat is Mimas, the giant born to slay Hephaestus. Piper warns Annabeth not to think their way out, and instead rely on their emotions. Using her knife, Piper is able to kill Mimas by having the statue of Phobos fall on his head.
The final ingredient lies on the island of Delos, the home of Apollo and Artemis. Apollo reveals that on Delos, the two aren't incapacitated with pain from the Greek-Roman schism. The final ingredient is a yellow daisy, known as the curse of Delos because when the twins were born, the island was so happy, it covered itself in yellow flowers and rooted itself in the ground. After Apollo gives Leo the daisy, Leo tells Frank and Hazel his master plan.
With all the ingredients, Leo, Piper, and Jason proceed to Asclepius' office. He gives Jason glasses and has his snake Spike make the physician's cure. Asclepius also notes that Piper had broken her arm after falling off a horse at age six and knows that Leo knows that he will have to die. He says it can be either injected or swallowed. Leo secretly keeps the real vial while Hazel manipulates the Mist so Piper thinks she has the real one.
Leo puts the vial in a needle and installs it in Festus. When Leo is killed by the blast that defeats Gaea, Festus is blown far away but manages to inject Leo's body with the Physician's Cure, and Leo is resurrected.