Another study of Tysabri, which included 2,248 patients with either Crohn's disease, multiple sclerosis or rheumatoid arthritis, found the risk of developing the PML brain infection was very low, according to Dr. William Sandborn of the Mayo Clinic, who led the study. Before the voluntary recall of the drug, three patients were identified with the infection, and four more infections were initially reported in post-marketing reviews. Sandborn's analysis, in which his team checked spinal fluid and blood, found no additional cases among the patients studied, and the four post-marketing cases did not have the infection after all.
http://www.forbes.com/forbeslife/health/feeds/hscout/2006/05/23/hscout532863.html