A second study in mice used a neurovirulent A/WSN/33 strain of H1N1 influenza A virus administered intranasally, resulting in infection of olfactory neurons and anterograde axonal transport to the olfactory bulbs (Aronsson et al., 2003). Using immunodefective mutant mice to examine how the immune system controls the spread and persistence of the virus, the authors showed that an absence of T and B cells allowed the infection to spread fully into the brain.