JC virus infection The cause of<unk>  is a type of polyomavirus called the JC virus (JCV), after the initials of the person (John Cunningham) from whose tissue the virus was first successfully cultured. Recent publications indicate 39 to 58% of the general population are seropositive for antibodies to JCV, indicating current or previous infection with the virus. Other publications put the percentage at 70 to 90% of the general population. JCV causes persistent asymptomatic infection in about one-third of the adult population, based on viral shedding into the urine from the site of asymptomatic infection in the kidney. The virus causes disease only when the immune system has been severely weakened. Immunosuppression<unk>  is most common in people with HIV1 infection; prior to the advent of effective antiretroviral therapy, as many as 5% of people with AIDS eventually developed<unk> . Why<unk>  occurs more frequently in people with AIDS than in other immunosuppressive conditions is unclear; some research suggests the effects of HIV on brain tissue, or on JCV itself, make JCV more likely to become active in the brain and increase its damaging inflammatory effects.<unk>  can still occur in people on immunosuppressive therapy, such as efalizumab, belatacept, and various transplant drugs, which are meant to weaken the immune system. Multiple sclerosis medications Natalizumab (Tysabri) was approved in 2004 by the FDA for the treatment of multiple sclerosis (MS). It supposedly works by preventing white blood cells from entering the brain. It was subsequently withdrawn from the market by its manufacturer after it was linked with three cases of<unk> . All three initial cases were taking natalizumab in combination with interferon beta-1a. After a safety review, the drug was returned to the market in 2006 as a monotherapy for MS under a special prescription program. As of May 2011, over 130 cases of<unk>  had been reported in MS patients, all in patients who had taken natalizumab for more than a year. While none of them had taken the drug in combination with other disease-modifying treatments, previous use of MS treatments increases the risk of<unk>  between three- and four-fold. The estimated prevalence of<unk>  in MS is 1.5 cases per thousand natalizumab users. Around 20% of MS patients with<unk>  die, and most of the rest are very disabled. One case study describes an MS patient who, during a 4-year course of dimethyl fumarate, developed<unk>  and died. Fingolimod (Gilenya) was approved in 2010 by the FDA for MS. In 2015, the first case of<unk>  and a case of "probable<unk> " were reported by two Gilenya users that could not be tied to previous immunosuppressant therapies. These new cases are now being added to the drug information sheet included with every prescription (i.e. the "drug label").
Progressive multifocal leukoencephalopathy
Toxic leukoencephalopathy
Tuberculous meningitis
Granulomatous meningoencephalitis
Herpesviral encephalitis
Malignant multiple sclerosis
Drug-induced aseptic meningitis
Polioencephalitis