People living in Southern California have been falling ill from a mysterious fungus known as Cryptococcus grattii, which can cause deadly infections — particularly in HIV/AIDS patients with compromised immune systems — for years.

The fungus is mysterious because it’s hard for scientists to pinpoint exactly where it comes from. Now, researchers, spearheaded by the science fair project of a 7th grader, have targeted certain California trees as the culprits.

Elan Filler was a 7th grader when she came up with the idea for a science fair project: Find the trees that cause the fungal infections. Previously, scientists had assumed eucalyptus trees were to blame, since they were a home for the fungus in Australia — but no one was ever able to find it in California. Douglas fir trees were also considered a suspect, since a different form of the fungus was growing on them in the Pacific Northwest, but no such trees exist in L.A.

“We had a good idea that the fungus was going to be associated with trees,” said Deborah Springer, a postdoctoral fellow at Duke University, according to NPR Atlanta. “We just didn’t know what trees.”

Elan was lucky to have a dad who happened to be an infectious disease specialist at the University of California (Dr. Scott Filler), who got his daughter in contact with Springer, who had been studying C. gattii for some time. With Springer’s help, Elan traveled around L.A. swabbing tree trunks and growing out fungus in Petri dishes. She found that up to three trees were to blame: Canary Island pine, New Zealand pohutukawa and American sweet gum. The fungus that grew on these trees matched almost perfectly with patients infected with C. gattii.

The results were published in PLOS Pathogens, with Elan Filler listed as an author. Springer advises anyone with compromised immune systems, such as people with HIV/AIDS or cancer, to be aware of C. gattii infections. “When you travel and you’re exposed to these reservoirs, you have the potential to be vulnerable to these infections,” Springer said. If the disease is caught early, however, doctors can treat cryptococcosis and save patients’ lives.

Source: Springer D, Billmyre R, Filler E, Voelz K, Pursall R, Mieczkowski P. Cryptococcus gattii VGIII Isolates Causing Infections In HIV/AIDS Patients in Southern California: Identification of the Local Environmental Source as Arboreal. PLOS Pathogens. 2014.