I have the JC Virus, and I also take Tecfidera. My neurologists keeps a very close eye on me, he does regular blood tests to make sure my blood cell count doesn't get low or to high. I haven't had any problems with it so far. The JC Virus does increase the risks of things, but as long as you stay in good contact with your MS doctor and do the blood work and of course your MRI's, you should be fine. Tecfidera put me in remission and has not given me any problems