More interesting info!

France has had NO cases of PML. Their medical prerequisites require them to test for CD4/CD8 counts to check the status of each patient's immune function and they do it to every patient. If the count is too low, no Tysabri until the count is normal. So, if a patient has a lowered immune system due to using another drug or anything else, no Tysabri.

JCV carriers:

"JC Virus has been used as a genetic marker to infer early human migration patterns. But new analyses show that it may be an unreliable indicator: this ubiquitous virus evolves much faster than previously thought and its evolutionary trees do not reflect known patterns of human population divergence.
Most of us contract JC virus (JCV) during childhood, from our parents. Only rarely does infection cause disease; mostly, it remains unnoticed, and we pass the virus on to our children in turn. JCV has at least 14 genetically distinct subtypes, some of which are associated with specific human populations — for example, type 7A predominates in Southeast Asians, types 3 and 6 in Africans, and types 1 and 4 in Europeans.
The parent-child transmisison route, together with the population specificity of particular subtypes, has led some researchers to use JCV as a genetic marker for tracing early human migration patterns. Furthermore, rates of JCV evolution have been inferred by assuming co-divergence with human populations over the last 200,000 years.
However, Laura Shackelton, Eddie Holmes and collaborators report that JCV and human evolution are not tightly coupled after all. When the researchers examined JCV from a variety of human populations worldwide, as well as mitochondrial genomes from the same set of human populations, they discovered that:
JCV is evolving much faster than previously thought
Much of JCV's genetic variability stems from a population expansion starting around 350 years ago, much later than early human migrations;
Evolutionary "trees" for JCV do not map at all closely onto the evolutionary history of humans inferred from mitochondrial sequences"

From a study published in the Journal of Virology

This makes me wonder if only certain subtypes of JCV are prone to developing into PML.

So many questions, so little known...sigh