The number of cases of progressive multifocal leukoencephalopathy, or PML, in patients taking Biogen Idec (Nasdaq: BIIB) and Elan's (NYSE: ELN) Tysabri increased by nearly 12% in less than a month. Annualize that out, and it isn't a pretty picture.

The latest monthly update listed 10 new cases of PML, bringing the total count to 95. The brain infection has killed 20 of the 95 patients that have been stricken while taking the multiple sclerosis drug.

As the number of patients taking Tysabri increases, we're bound to see the PML cases increase, but the update for January was considerably higher than the last two updates; November had four new cases and December had six.

Of course, one month doesn't make a trend, and Biogen points out the overall rate is still fairly low at 1.16 per 1,000 patients.

The question is where does it cross the line from being an acceptable risk to being an unacceptable one. Tysabri is the best multiple sclerosis drug out there, but there's a point at which doctors and patients won't think the benefits outweigh the risk, and they'll head to other treatments such as Teva Pharmaceutical's (Nasdaq: TEVA) Copaxone, Novartis' (NYSE: NVS) Gilenya, and Rebif, which is sold by Merck KGaA and Pfizer (NYSE: PFE).

The Tysabri duo could also lose sales if patients decide to go on drug holidays. The risk of getting PML increases the longer a patient is on Tysabri. Patients may try to delay increasing the risk by taking a break from treatment, cutting into Biogen and Elan's sales.

The companies can't do anything to affect the rate of PML, but they can help patients identify risk factors that increase the likelihood of getting PML. If the companies encourage patients who are least likely to get PML to take Tysabri, the overall rate should decrease.

In that vein, the companies are developing a test for the virus that causes PML. Patients who don't have the dormant virus in their systems are less likely to get PML when Tysabri deadens their immune systems.

Biogen and Elan better hurry, before the marketing message is permanently damaged.

Keep track of Biogen and Elan as they deal with PML. Click on their names to add them to My Watchlist, which will help you keep track of all our Foolish analysis on the companies.

Pfizer is a Motley Fool Inside Value selection. Elan is a Motley Fool Rule Breakers pick. Novartis is a Motley Fool Global Gains recommendation. The Fool owns shares of Teva Pharmaceutical Industries. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.