Mayfly is a highly carcinogenic alien-derived transformative bioagent.
When injected into a human, Mayfly continuously generates exotic tumors in the brain, resulting in death within 24 hours. However, in 1 out of 500 humans, the alien cancer cells turn them into supragenius-level savants for the short time until they die. Typical Mayfly users are people who are desperate and/or already dying.
The aspiring supervillain Manny used Mayfly.