Dianne Feinstein, a Democratic senator from California, returned to the Capitol last week after spending more than two months recovering from shingles.
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.
Post-shingles encephalitis can cause headache, fever, sensitivity to light, vomiting, confusion, a stiff neck or even seizures.
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.