Seasonal flu places a heavy burden on both human populations and health care ser-
vices every year, warranting permanent surveillance. Online-based surveillance mod-
els harness the collective online search activity of flu-infected individuals to provide
real-time monitoring of flu activity. These models assume that most flu-related online
behavior is motivated by a flu infection. However, when the flu pandemic emerged
in 2009 it resulted in abnormal search behaviors that confounded these models, as
several reasons, beyond infection, can motivate individuals to seek flu information.
In practice, and despite their potential, current models cannot distinguish whether
such activity is related with actual flu infection or not, rendering them useless,...