I was scared to start Tysabri as well, but I had my 1st treatment earlier this month.
I am going downhill. Slowly (not slow enough) and surely downhill. I have been on other meds to no avail. So my choices were 1) do nothing 2) take meds that have proven ineffective 3) take Tysabri.
What other choice did I have? I started Tysabri. At least there is a little hope again.
I have a very good friend that is a neuroradiologist. He concvinced me by explaining that the incidence of PML is less than problems caused by everyday medications that people take for granted like high cholesterol meds, etc.