The probability of developing PML when treated with Tysabri depends on the presence or absence of anti-JCV antibodies in the blood (JCV-positive or JCV-negative), the level of anti-JCV antibodies in those patients that are JCV-positive (JCV-index), treatment duration and prior use of immunosupressants (e.g. mitoxantrone, cyclophosphamide, azathioprine).
The John Cunningham virus (JCV) is a common infection. About 50–70% of the general population is estimated to have been infected by JCV, but the number varies greatly from country to country. However, most people exposed to the virus are not aware of it, since the virus is normally kept under control by the immune system.
How to assess probability
Everything you do and everything you deal with have potential risks. Living your life is to constantly assess risks in relation to benefits and necessity. In order to make it easier for ourselves to make important decisions, we generalize and simplify. When probabilities become big numbers, sometimes hard to understand, we sometimes to fall back on emotional decisions and to act in a way that we usually do.