Medusa virus

The Medusa Virus was first mentioned as "Project Medusa" by the real Hank Henshaw when he trespassed into Superman's Fortress of Solitude, and stole the files, using Kara's blood in "The Darkest Place"

The Test
The Real Hank Henshaw is a bitter man who hates all aliens, believing that they should be dead, or in space, away from earth rather than being on earth. After stealing the files from the fortress of Solitude, he goes to the dive bar to release the test of the virus. Mon El seemed to be present at the bar, and follows Henshaw out of the bar when the virus goes off. All aliens in the dive bar are instantly killed, declaring to cadmus that the virus is ready for the next phase

Mon-El
Mon El, after leaving the bar, gets into an altercation with Hank Henshaw as the virus kills the aliens at the bar. Once back at the D.E.O. Mon El is put into confinement, as J'onn believes that he could be residually effected. However, Mon-El didn't have to be alone. Kara was also put into confinement, as Alex didn't want her to be effected by this virus. As the two play monopoly, Mon-El is seen to have been residually effected.

While effected, Mon-El and Kara have a heart to heart, and he kisses her. Alex brings in her mother, Eliza Danvers, to help as she is a doctor that deals with bio engineering. With Eliza's help, they're able to determine that Medusa's next part is to be airborne but needs an isotope that L-Corp currently has.

Zor-El and the Ruling Counsel
According to Zor-El's holographic projection to Kara in the Fortress of Solitude, during "Medusa," The Medusa Virus was created by Zor-El along with his brother, Jor-El, by the Ruling Counsel if Krypton were to ever be invaded by any outside or "other worldly" aliens. The Medusa Virus was created to save Kryptonian lives on Krypton, regardless of the virus' implications on any other planets. Kara doesn't agree nor does she condone her father's actions.

Lena & Lillian Luthor
Supergirl tries to warn Lena about her mother, Lillian Luthor, and her activities as the head scientist of Cadmus Labs. Instead of believing Supergirl about her, she claims that Supergirl is lying about her mother, but then concocts a scheme of her own to use her mother. At first she plays along and offers the isotope to her mother, and they plan to release the Medusa Virus in the air together. Unknowingly Lillian, Lena switched the isotopes, and makes the Medusa Virus useless, in addition of calling the police and arresting Lillian Luthor.

Eliza Danvers
Because of the virus, Eliza was able to isolate a cure for Mon-El. As he recovers, Kara and Mon-El both played dumb about the kiss, as the both of them make believe it was because he was close to death. In addition to curing Mon-El, Eliza also used the reversed virus to help J'onn from turning into a White Martian.