Scientists may have uncovered a way to track the ever-evolving flu virus buried in 10-year-old snot. In an effort to understand how the flu virus rapidly mutates — which leaves scientists constantly scrambling to come up with a new flu vaccine — researchers decided to study four cancer patients’ snot, which had been collected a decade ago and frozen.

Because cancer patients tend to come down with the flu for a longer period of time than healthy individuals, the scientists had a longer window of time to observe the mutating virus. In healthy humans, the immune system typically eradicates the flu virus before it undergoes too much mutation, making it harder to track what is coming next in the flu’s evolution.

The team “deep sequenced for all the different mutants of one strain of flu called H3N2,” Wired reports. Initially, biochemist Jesse Bloom said the research team expected “the type of evolution that flu undergoes in any individuals … might end up being very idiosyncratic.”