However, the FIMS hypothesis proposes that mammals had several possible adaptations against these fungal invaders that other animals did not. In modern birds, airsacculitis can be caused by fungal infections such as aspergillosis , but also by bacterial infections. The FIMS hypothesis also has a contradiction at its heart, said Jingmai O'Connor , a paleontologist at the Field Museum in Chicago. "So secondary fungal infections are certainly possible," Jimenez said, referring to infections that take root after the body's defenses are weakened. But skeptics argue that even with these updates in the FIMS hypothesis, more evidence is needed to show it's not just possible but likely.