Long technical explanation?

The theory is that the JC virus causes PML. It is believed that approximately 50%+- of the world's population carries the JC virus, which lies dormant in the kidneys. When the immune system is depressed, it does not keep the JC virus in check and it travels to the brain and munches on myelin, causing PML (progressive multifocal leukoencephalopathy)

We don't know how people are exposed to the JC virus and we don't know why it goes crazy in some people and not in others. Most of what is known about the clinical presentation of the JC virus and PML was learned from AIDS patients. There is no vaccine or any way to rid the body of the virus at this time, although an attempt is being made to develop a vaccine.

There is a new assay, which is going into study in the next few weeks. Many Tysabri patients are signed up for the study. The assay checks to see if the patient has been exposed to the JC virus. It has a false negative rate of 2.5%+-. The results will be released to the doctors and
to the patients via their docs.

Patients can reduce the risk of PML by being vigilant and can go off Tysabri for a few months and their immune system goes back to normal. Theory is that the clock resets when you restart.