When the Skrulls invaded San Francisco, they learned that the X-men were the city’s protectors. After a series of back and forth skirmishes, the Skrull commander decided to take a few civilian hostages to force Cyclops and the X-men to surrender. What Cyclops did instead was have Beast modify the legacy virus and adapt it to Skrull physiology. Basically, Cyclops used biological warfare against the aliens and offered them the chance to surrender in exchange for the cure.
From – Secret Invasion – X-Men #4