Dianne Feinstein, a Democratic senator from California, returned to the Capitol last week after spending more than two months recovering from shingles. The disease, often characterized by a painful rash, is triggered by the same virus that causes chickenpox, which stays in people’s bodies for life and, years later, can become reactivated.

For Ms. Feinstein, 89, the virus also brought on a previously unreported case of encephalitis, a rare but potentially debilitating complication in which the brain swells. The condition is often caused by an infection or an immune response.

What are the symptoms of encephalitis?

Post-shingles encephalitis can cause headache, fever, sensitivity to light, vomiting, confusion, a stiff neck or even seizures.

It can also leave some patients with more lasting problems. Those include memory or language trouble, sleep disorders, mood disorders, walking difficulty and other cognitive problems. Older patients tend to have the most trouble recovering.