U.S. President Joe Biden, tested positive for COVID-19 on Thursday, is experiencing mild symptoms and has begun taking Paxlovid, an antiviral pill treatment.

He is reporting a runny nose, fatigue and an occasional dry cough, according to a memo from his doctor Kevin O' Connor. 

A look at what's known about Biden and his coronavirus infection:

What kind of test did he use?

Biden is routinely screened for coronavirus. His infection was detected first with an antigen test, the same type that many Americans use at home. It was then confirmed with a PCR test, the president's doctor said. The president's last previous test for COVID-19 was Tuesday, when he had a negative test result.

Biden got two doses of the Pfizer vaccine shortly before taking office, a first booster shot in September and an additional dose March 30.

Is he getting treatment?

Biden is taking Paxlovid, which was authorized in the U.S. late last year. In older people and other high-risk patients, the drug was shown to reduce the chances of hospitalization or death from COVID-19. 

He will work in isolation for at least five days and until he tests negative, the White House said. Once he tests negative, he will return to in-person work. The White House said it will provide a daily update on the president's status.

How does the virus affect older people?

At age 79, Biden is in a high-risk group for severe illness. About eight in 10 COVID-19 deaths occur in people over age 65, with risk increasing with age. Also among the most vulnerable are those with other health issues such as diabetes and heart conditions.

Did he get the BA.5 variant? 

It's unclear which variant Biden has contracted, though a sample has been sent for genetic sequencing to determine that. The dominant virus variant in the U.S. is the Omicron mutant BA.5, which is also prominent worldwide.

The variant accounted for three-quarters of new infections reported in the U.S. last week. It's also been gaining ground globally, making up more than half of sequenced Omicron cases. Experts say BA.5 is one of the most transmissible variants yet but they haven't seen an increase in severity compared with previous Omicron variants.

Where did he catch it?

That's unclear. Symptoms can start two days to two weeks after exposure to the virus. Biden has kept a busy travel schedule lately. During his recent Middle East trip, the U.S. president was seen fist-bumping, handshaking and even occasionally hugging. He returned to Washington late Saturday and kept a low profile for the next three days, leaving briefly to attend church and for a Tuesday appearance outside the White House with Ukraine's first lady. He traveled to Massachusetts on Wednesday to announce modest new steps to combat climate change.