While PML (progressive multifocal leukoencephalopathy) sounds scary, advances are being made in its diagnosis and treatment.

The first question is: who is at risk to get PML? The JC virus that causes PML is a common virus that many of us have had since childhood without even knowing it. Most people (with healthy immune systems) don't develop PML because the body's natural immune defenses are able to keep the virus in check. White blood cells (lymphocytes, leukocytes) constantly scavenge the body looking for viruses and stop them at the source. It is only in people where the immune system is suppressed that the JC virus can get out of hand and cause an infection in the brain. This infection is known as PML.

In the past few years, we have seen PML occur in individuals with multiple sclerosis who take a new medication, Natalizumab (also known as Tysabri®). Although MS affects the immune system, the immune system of an MS patient becomes overactive and attacks the central nervous system (CNS). People with MS are still able to fight off infections; simply having MS does not place someone at risk for developing PML.

Natalizumab helps people with MS by preventing misdirected lymphocytes from entering the CNS through the blood brain barrier. This prevents misdirected lymphocytes from damaging the covering of nerves (known as myelin) and the nerves themselves (axons) in the CNS. However, if there aren't enough lymphoctytes getting into the CNS - which is good for MS symptoms - it may limit the immune system's ability to scavenge and destroying viruses. This is why for the first time in MS, we saw patients on Natalizumab develop an infection of the JC virus in the brain - PML.

Fortunately, only a minority of people on Natalizumab are at risk for developing PML and we are learning more about the factors that would increase or decrease risk. The first question is whether an individual has ever been exposed to the JC virus. If you don't have the JC virus in your body, then the JC virus shouldn't be able to get out of control and spread in the brain. We used to think that 80 percent of the population has been exposed to the JC virus; more recent research (including research sponsored by Natalizumab's manufacturer) suggests the actual ratio is more like 50 to 60 percent of people have been exposed to the virus.

Remember, the JC virus does not normally cause unique symptoms, so you can't tell simply from your history of flu-like episodes if you were exposed to it in the past or not. There is a simple blood test in development that can tell you whether you have been exposed to the JC virus. The blood test measures whether you have antibodies to the JC virus. These would appear as your body makes antibodies in response to foreign invaders, such as viruses.

The thinking is that if you have not been exposed to the JC virus, and the virus isn't in your body, you should be at low risk to develop PML even if you have MS and take Natalizumab. The JC virus is generally a virus that you get in childhood, so if your lab test shows that you have never gotten the virus, then you are probably not at risk for getting it now - but doctors are still researching this point.