The John Cunningham virus, known more commonly as the JC virus, is a very common virus in the United States. According to the World Journal of Neurosciences, between 70 and 90 percent of people in the world have the virus. The average person carrying the JC virus will never know and is unlikely to experience any side effects.
However, that’s not the case for a small percentage of individuals with multiple sclerosis (MS). The JC virus can be activated when a person’s immune system is compromised because of disease or immunosuppressive medication.
The virus can then be carried into the brain. It infects the white matter of the brain and attacks the cells responsible for making myelin, the protective coating that covers and protects nerve cells. This infection is called progressive multifocal leukoencephalopathy (PML). PML can be disabling, even fatal.

The role of immune-suppressing drugs

The JC virus often attacks when a person’s immune system is at its weakest. A weakened immune system can no longer fight off invading viruses. It’s the perfect opportunity for the JC virus to awaken, cross the blood-brain barrier, and begin attacking the brain. People with MS are at an increased risk for PML because their immune system is often compromised as a result of the condition.
Further compounding the problem, several medications used to treat the symptoms of MS can also compromise the immune system. Immunosuppressant medications can increase the likelihood that a person with MS will develop PML after exposure to the JC virus. These immunosuppressant medications may include: