Neurodegenerative diseases were once considered disorders of the
mind, rooted in psychology. Now viruses rank among the
environmental factors thought to trigger brain-ravaging diseases
such as multiple sclerosis (MS) and Alzheimer’s disease. Human
herpesvirus-6 (HHV-6), in particular, has been linked to MS in
past studies. Neuroscientist Steven Jacobson and his colleagues
at the National Institute of Neurological Disorders and Stroke
have determined that the virus makes its entry to the human
brain through the olfactory pathway, right along with the odors
wafting into our nose. The researchers tested samples of brain
cells from people with MS and healthy control subjects and found
evidence of the virus in the olfactory bulb in both groups.
Infection via the nasal passage is probably quite common, as is
harboring a dormant reservoir of HHV-6, but in people with MS,
the virus is active. Genetics and other unknown environmental
factors probably determine the likelihood of the virus
reactivating once inside the b