Senate Democrats technically hold a 51-49 majority over the Republicans, but as has been seen this week, that thin advantage is rather tenuous and illusory in reality.

Democratic California Sen. Dianne Feinstein revealed that she will be hospitalized and treated for shingles for the foreseeable future, Axios reported.

Her being hospitalized obviously prevents the long-serving senator from being present in the Senate for roll call votes, floor debates and speeches, and committee hearings.

Hospitalized with shingles

"I was diagnosed over the February recess with a case of the shingles," Sen. Feinstein said in a statement through a spokesperson on Thursday, according to the Daily Mail.

"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," the 89-year-old senator added.

According to the Mayo Clinic, shingles are caused by the same virus as chickenpox, known as varicella-zoster, and that incurable virus remains in the body forever once it has been contracted.

Shingles are not fatal but can cause painful rashes and blisters, typically around the torso, and is treatable with a variety of drugs to ease the pain and speed the healing of the blisters and rashes.

Absence follows recent retirement announcement

The revelation on Thursday that Sen. Feinstein was hospitalized to receive treatment for shingles helps explain her absence from the Senate over the past few days, according to NBC News.

On Wednesday, when asked why the senator had been missing this week, Feinstein's office had simply said that she was "dealing with a health matter" and fully intended to "return to Washington soon."

Her absence this week was questioned after the senior senator from California missed a handful of crucial roll call votes on the Senate floor as well as an important Senate Judiciary Committee hearing on Wednesday with Attorney General Merrick Garland, according to The Washington Post.

It also comes shortly after it was announced that Feinstein will not seek re-election to another term in office in 2024 and instead will retire at the conclusion of the current session.

That announcement came amid increasing media scrutiny over her advanced age and an apparent decline in her cognitive capabilities, which may have been the reason why Feinstein declined the opportunity to serve as president pro tempore during this session, a position of honor that typically goes to the longest-serving senator of the majority party and is third in the line of succession for the presidency behind the vice president and House speaker.

Senate majority in question

According to the Washington Examiner, Sen. Feinstein's hospitalization further imperils the Senate Democratic majority that had already been placed at risk when Democratic Pennsylvania Sen. John Fetterman was hospitalized last month to receive treatment for clinical depression.

Meanwhile, Democratic Oregon Sen. Jeff Merkley has also been absent this week as he deals with the death of his mother – but the Senate Republicans have also been short a member this week as GOP Idaho Sen. Mike Crapo has been absent while recovering from a coronavirus infection.

That means, rather than a 51-49 majority for Democrats, the Senate has effectively been split 48-48 this week, which required Vice President Kamala Harris to cast a tie-breaking vote on a few occasions.

It also means that the legislative agenda for Democrats and President Joe Biden is dead in the water right now, as they can really only hope to muster the numbers to confirm some of Biden's less-controversial executive and judicial nominees.

The situation has also presented a small window of opportunity for Senate Republicans, despite their minority status –and likely with the assistance of a few more moderate Senate Democrats – to actually pass a few items from their own legislative priorities list, though Biden has vowed to veto most anything that the GOP might manage to land on his desk in the White House.