Three is not a lucky number for Biogen Idec (NASDAQ:BIIB) and Elan (NYSE:ELN).

That's the number of cases of progressive multifocal leukoencephalopathy (PML) that have been reported since their multiple sclerosis drug Tysabri made its return. It was put back on the market in 2006, after having been pulled a year earlier over reports of PML in Tysabri patients.

The third case of the often-deadly brain disorder was reported yesterday, but are three cases really that much worse than two? I really don't think so.

Another case was bound to pop up. As more patients use Tysabri, it becomes more likely that cases will be reported -- that's probability at work. The Food and Drug Administration didn't seem all that worried about the two cases reported in July. Other drugs -- like Rituxan, from Genentech (NYSE:DNA), and Myfortic, from Biogen and Novartis (NYSE:NVS), have also been linked to PML. So I doubt that the additional case is going to get Tysabri pulled from the market. The only real question is whether an additional case will affect sales of Tysabri.

The growth curve for Tysabri sales has already begun to slow as patients switch to other multiple sclerosis drugs -- including Copaxone, from Teva Pharmaceutical's (NASDAQ:TEVA), or Rebif, from EMD Serono and Pfizer (NYSE:PFE) -- or stop therapy altogether. I doubt that the increase in the number of reported cases of PML is going to change the adoption rate for Tysabri. Given how well the drug works, I think patients will continue to switch as they fail on their current treatments and run out of options. Tysabri won't be the first choice for most multiple sclerosis patients, but it still has plenty of growth left in it.

I think investors are overreacting to the news. As a result, Biogen and Elan are both reaching prices that you just can't resist.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is a pick of the Income Investor and Inside Value newsletter services. The Fool owns shares of Pfizer and has a disclosure policy.