Shares of Biogen Idec Inc. (NASDAQ: BIIB) plummeted on news that its multiple-sclerosis drug Tysabri was linked to brain infections causing death, leaving 4.3% shareholder Carl Icahn with a hefty paper loss.

But Icahn didn't back down. Having already pushed for one unsuccessful effort to sell the company, and having lost a bid for three seats on the board of directors, Icahn filed a 13-D announcing that he had raised his stake to 6.1%, buying in the wake of the brain infection announcement.

As one of the smarter activist value investors going, Carl Icahn's moves are closely watched by investors looking to piggyback off his ideas.

But I'm not so sure about this one. What does Icahn know about Tysabri and brain infections anyway? Is he really in a position to make a judgment about the effect that the brain infection problems will have on the company? I doubt it.

Investors should be careful of following Icahn into Biogen -- so far his ideas haven't panned out, and he seems to have strayed far from his circle of competence.