California Democratic Senator Dianne Feinstein has been admitted to a hospital in San Francisco for treatment of shingles.
Ms. Feinstein, 89, was diagnosed in February with the virus, which causes a painful rash, and is expected to make a full recovery, she said in a statement.
Shingles often develops in adults with chicken pox. The rash can last and damage the nerve causing pain.
Ms Feinstein said last month she would not seek re-election.
According to medical experts, shingles is usually not life-threatening.
Ms. Feinstein has been in California since late February and has missed more than a dozen votes and two committee hearings since her illness began, her spokeswoman told the San Francisco Chronicle.
On Tuesday, his office announced that he would remain in California because he was “dealing with a health issue”.
“I was diagnosed with a case of shingles during the February break,” the senator said in a statement to BBC News on Thursday.
He added that he would “make a full recovery” and “hope to return to the Senate later this month.”
Ms. Feinstein, who was first elected in 1992, said last month that she would not seek re-election in 2024.
So far, three California members of the House of Representatives have announced bids to replace him.
Ms. Feinstein is not the only Senate Democrat who has been sidelined by health issues in recent weeks.
Senator John Fetterman is hospitalized for clinical depression.
Senator Bob Casey had prostate cancer surgery last month, but has since returned to work.
Democrats hold a narrow 51-49 lead in the chamber, meaning a shortage of senators reduces their ability to pass votes.
Twice this week U.S. Vice President Kamala Harris had to travel to the Capitol to exercise her ability to cast tie-breaking votes.
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