SAN FRANCISCO — U.S. Sen. Dianne Feinstein was receiving treatment for shingles at a San Francisco hospital, her spokesperson confirmed on Thursday.

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Feinstein, 89, the senior senator from California who has held the post since 1993, was diagnosed with the virus late last month, the San Francisco Chronicle reported.

Shingles causes a painful rash, according to the Mayo Clinic. It is generally not life-threatening and is the same virus that causes chickenpox.

“I was diagnosed over the February recess with a case of the shingles,” Feinstein said in a statement on Thursday. “I have been hospitalized and am receiving treatment in San Francisco and expect to make a full recovery. I hope to return to the Senate later this month.”

Feinstein, who turns 90 in June, is the oldest member of Congress, KNTV reported. She announced last month that she would not seek reelection in 2024.

So far three Democratic representatives -- Katie Porter, Adam Schiff and Barbara Lee -- have announced campaigns for the seat, according to the Chronicle.

Feinstein has been in California since late February and has missed a dozen votes and two committee hearings since she began feeling ill, spokesperson Adam Russell told the newspaper.

She serves on four Senate committees, which have held 12 public hearings and six closed hearings this year. She has attended nine of those 18 hearings, according to the Chronicle. She also sits on the intelligence committee.

She is expected to make a full recovery, KGO-TV reported.