The JC virus is a virus that lies dormant in about 80% of people's brain (http://en.wikipedia.org/wiki/JC_virus). This doesn't present a problem unless a person becomes immuno-compromised. If that happens, as with the AIDS population, the virus can activate into PML -- a quick moving, often fatal brain virus. The three people who got PML were severely compromised, not just Tysabri, but other immune suppressing meds too. (I don't know more details.) Since Tysabri was put back on the market in '06 as a regulated monotherapy, approximately 21,000 people are using the drug with no new complications of PML.
FinalMacGyver -- From the time I filled out my paperwork until I actually walked in for my first infusion, it was about 2.5 weeks. The labs nearby were not yet certified, so that was part of the problem. The prior-authorization (BlueCross/BlueShield) was quick -- maybe 3 days. My doctor's office was very thorough about submitting all the necessary documentation to prove that I have not responded to any other marketed drug. There is really no way that my insurance could deny the drug, from my perspective. Thankfully, there wasn't a fight. (I did have to fight with them in the past when I was getting Remicade every 4 weeks!)
The infusion is an hour... There was about a half hour up front (IV insertion, sign forms, etc.), the one hour infusion followed by a one hour observation (IV left in). So, total was 2.5 hours.
Well I'm anxious to start it. I have been off of HUMIRA for a little over a month due to them trying to get it approved through insurance for once a week rather than every other. The only thing that helps me is steroids, so I hope this gets approved a started quick as ill be completely off steroids in a month.