While cleaning out files I found an excellent synopsis of JCV and PML risk.
I do not know where I got it. I am sorry I cannot give credit to anyone for it, but whoever it was, if they see this-THANK YOU!

PML (best guess and best evidence) is a multi step process....
First, you must get the 'standard' virus which hangs out in the kidneys and lymph tissue. During this short time, you will be JCV DNA positive. From this point on, you will also be JCV ANTIBODY (IgG) positive (after 3 to 4 weeks).

After primary infection, it goes dormant but may flare up periodically. When flares up you could be DNA positive.
Also, when it flares up, it may mutate.
When mutated, the infection usually is quickly controlled.
However, some mutated versions can go to the brain.

If in a brain with a 'normal' immune system, PML will not develop as the virus is quickly neutralized by resident immune cells.

With Tysabri (or Rituxan or some other drugs), there is less ability to control the mutated virus in the brain and PML may develop.

So, a person who was exposed to JCV will be JCV positive for the rest of their life but be JCV DNA positive only short rare intervals.....and when DNA positive, it does not mean the mutated form which may cause PML.

The STRATIFY study is for the JCV Antibody.
If JCV Ab positive, your risk of PML after 18-24 months is about 1:400.
If JCV Ab negative, then your chance is very small (probably 1:10,000 range).

The reason risk is not zero is because no laboratory test is perfect and because if you are negative, you have a small chance of getting the standard (non-mutated) virus infection in any given year and will then be JCV Ab positive,

If you get the standard virus, all other steps must happen to get PML.