Russian President Vladimir Putin’s former adviser is reportedly “in a bad state” after suffering from a rare immune disorder.

According to Ksenia Sobchak, a Russian journalist and political activist, Anatoly Chubais, 67, was recently diagnosed with Guillain-Barre syndrome (GBS)—a rare disorder that attacks the nervous system.

Sobchak reported that she spoke with Chubais’ wife, who confirmed that her husband’s “state is unstable. He felt bad very suddenly. He began to stop feeling [his] arms and legs.”

She quoted Chubais, who explained his current state: “I was hospitalized in one of the European clinics with a diagnosis of Guillain-Barr syndrome. Condition of moderate severity, stable.”

GBS is a rare neurological disorder in which the immune system attacks the body’s nerves, causing muscle weakness and, in some cases, paralysis. According to the Centers for Disease Control and Prevention, most people diagnosed with GBS begin to recover within 2-3 weeks of the onset of symptoms, but it can also take as little as a few weeks or as long as a few years. However, a small percentage of those diagnosed experience permanent nerve damage (paralysis) and/or death. Common treatments include plasma exchange and high-dose immunoglobulin therapy.

Reuters reported that a European intelligence agency is investigating the case but has not yet released its findings. Meanwhile, some media and opposition activists speculated that Chubais had been poisoned, which was quickly debunked by an anonymous source. They said Chubais is “still in intensive care” with his wife by his side, and while he can’t walk, he can still talk.

“Remember that Chubais knows a lot about the inner machinery of [Putin’s] regime,” one Twitter user said. “If he started to ‘sing‘ there could be major repercussions.”