As part of a joint effort between the Science and Military Guilds, the Ruling Counsel of Krypton ordered scientist Zor-El to create a bioengineered virus known as the Medusa Virus. The virus was a fail-safe mechanism designed to protect Krypton from invasions by killing all life forms besides Kryptonians.
Stolen by Cadmus
In late 2016, a terrorist organization called Project Cadmus collected a sample of Supergirl's blood in order to gain access to the Fortress of Solitude. Hank Henshaw used the blood to access the files on Project Medusa.
After stealing the files from the fortress, Cadmus' leader, Lillian Luthor, altered the Medusa virus to ensure it wouldn't affect humans. Afterwards, Henshaw went to Al's Dive Bar to release a test sample of the virus, which immediately killed all the aliens inside. Mon-El was present at the bar as well, but wasn't immediately harmed by the virus.
Mon-El was placed in quarantine until the D.E.O. could understand what caused the outbreak at Al's Dive Bar. Additionally, Kara Danvers and J'onn J'onzz stayed inside the D.E.O. headquarters to protect themselves. Dr. Eliza Danvers was brought in to help study the virus due to her specialty in astrobiology. After checking up on Mon-El, Eliza revealed after studying a strand of the virus that it had originated from Krypton. Thus, since Mon-El's Daxamite physiology was similar to Kryptonians', he was able to fight off the virus longer, but would ultimately succumb to his illness without a cure.
Kara then flew to the Fortress of Solitude, where an A.I. of her father revealed the origin of the Medusa virus. She brought all the information to Eliza in hopes that a cure could be made to save Mon-El.
Launching the virus
Cadmus prepared to launch the Medusa virus airborne from the National City Port using Isotope 454 from L-Corp, which would disperse it all over National City. Supergirl tried to warn Lena Luthor about her adoptive mother, Lillian, revealing her as the head scientist of Cadmus who was planning to exterminate all extraterrestrial life on Earth. Lena then concocted a plan to stop Lillian.
Feigning her allegiance to Cadmus, Lena seemingly handed over Isotope 454 to Lillian and launched the rocket containing the virus. Supergirl attempted to stop the rocket, but failed. However, none of the aliens in the city were harmed; Lena had secretly switched out the isotope, rendering the virus inert. The police then arrived, having been called beforehand by Lena, and arrested Lillian.
Using a sample of the virus, Eliza was able to reverse-engineer it to create a cure, saving Mon-El's life. She also used the virus to eradicate the White Martian blood cells from J'onn's blood, restoring his Green Martian form.
The Medusa virus was designed to kill any species not native to Krypton as a last defense against invaders. When Project Cadmus reproduced the virus, they modified it so it also wouldn't harm humans. Daxamites can survive exposure to the virus longer than other species due to having near-similar physiologies to Kryptonians, but will ultimately die unless cured.
The virus cannot be transmitted between organisms and only infects its targets with direct contact when aerosolized. Once contracted, the virus poisons and kills its victims in seconds.
The virus can also destroy alien species on a molecular level, as shown when Eliza Danvers used a sample of the virus to destroy the White Martian cells in J'onn J'onzz's bloodstream, preventing the Green Martian from completely transitioning into a White.