LONDON — Anna Netrebko, the star Russian soprano, is in the hospital with pneumonia caused by the coronavirus, she said in an Instagram post on Thursday.

“I have been in the hospital for 5 days with Covid-19 and will recover soon,” Ms. Netrebko wrote, adding: “Everything will be fine! The devil is not so terrible as it is described.”

She was admitted to a Moscow hospital just days after performing at the city’s Bolshoi Theater with another singer who later tested positive for the virus. Scientists have identified singing indoors as a risky activity, and the news will cause concern among the world’s opera companies, which had been closely watching the starry Bolshoi performances as they forged ahead.

Maxim Berin, Ms. Netrebko’s Russian manager, said in a telephone interview that the soprano had been unwell since Sept. 9, but he said she was now recovering.

“Doctors told me everything is fine, and now Anna has started to sing in hospital, which is why she made the post,” he said of her Instagram announcement.

Ms. Netrebko, who lives in Vienna, had been in Russia since the start of September, Mr. Berin said. She performed with pianist Denis Matsuev, among others, in Moscow on Sept. 2. Then, on Sept. 6 and 8, she appeared at the Bolshoi in two performances of Verdi’s “Don Carlo,” the first opera at the venue since it was shuttered in March because of the pandemic.

The soprano was scheduled to appear there for a third time on Sept. 10, but the company canceled that performance after the bass Ildar Abdrazakov became unwell with a slight fever. He later tested positive for the coronavirus.

The Bolshoi immediately tested those who had been in close contact with Mr. Abdrazakov, Katerina Novikova, a spokeswoman for theater, said in a telephone interview. That included all of the other soloists in “Don Carlo,” as well as employees working in makeup and wardrobe. Three tested positive and were ordered to isolate for two weeks, Ms. Novikova said, but Ms. Netrebko was not among them.

Mr. Berin said that Ms. Netrebko had been tested “several times” after the Bolshoi outbreak and had been negative each time. The virus was confirmed only when she was at the hospital, he added.

Ms. Netrebko used the Instagram post largely to defend her decision to go to back to work. “I had two choices,” she wrote: to “stay at home and be afraid of getting infected” or “start working, traveling around, performing at the risk of getting sick.”

“I unconditionally chose 2 and do not regret anything,” she said.

The incident is not the first to affect a Russian cultural institution since music and dance performances resumed shortly after President Vladimir V. Putin declared in July that the battle against the coronavirus had been won. Last month, the Mariinsky Ballet in St. Petersburg suspended all performances, classes and rehearsals after over 30 people in the company contracted the virus.

Those infections occurred despite the ballet’s adopting a wide range of measures to keep staff members safe, such as regular testing, temperature checks and mandatory mask wearing. Similar safety measures were in place at the Bolshoi, Ms. Novikova said, though regular testing is not.

Other countries in Europe have also seen outbreaks since performances resumed. In Austria, the health authorities reported a cluster of 46 infections after an operetta performance by students at the Vienna Music and Arts University early this month.

Among those infected was a member of the Vienna State Opera who was at the event, Bernhard Mayer-Rohonczy, a spokesman for the university, said in a telephone interview. Two other State Opera employees became infected in a rehearsal with that person, the opera company said in a statement, and all three have been in quarantine since.

On Wednesday, the Budapest Operetta and Musical Theater in Hungary also canceled all performances after an artist tested positive for the coronavirus, according to a statement on its website.

But regardless of the outbreaks, it seems activity is unlikely to slow down. Ms. Novikova of the Bolshoi said that the theater was doing all it could to keep people safe, but that singers and musicians “really want to work.”

“We can’t wait, sitting at home,” she said.

In her Instagram post, Ms. Netrebko seemed to agree. “Do not worry!” she wrote. “You won’t take me so easily.”