"Good Morning America" anchor George Stephanopoulos has tested positive for COVID-19, the illness caused by the novel coronavirus.

Stephanopoulos announced his diagnosis Monday on "GMA," nearly two weeks after his wife, actress and bestselling author Ali Wentworth, confirmed she tested positive for COVID-19.

Stephanopoulos, who shares two daughters with Wentworth, has been his wife's caregiver throughout her battle with COVID-19. Wentworth self-isolated in a room in the family's New York home at the start of her symptoms in order to try to avoid spreading the virus.

Stephanopoulos said it was "no surprise" that he tested positive for COVID-19, and revealed that, unlike Wentworth, he has been asymptomatic.

"I’ve never had a fever, never had chills, never had a headache, never had a cough, never had shortness of breath," he said. "I’m feeling great."

As many as one in four people in the U.S. may have COVID-19 and not show any symptoms, according to estimates from the U.S. Centers for Disease Control and Prevention (CDC).

Dr. Jennifer Ashton, ABC News chief medical correspondent, said that new data from Iceland -- where there is a smaller population and testing is more prevalent -- shows that 50% of people who are infected with COVID-19 show no symptoms at all.

"That is part of why it’s so difficult to contain this virus," she said Monday on "GMA." "We have to remember this virus is just about four months old so literally we’re learning things about the way it behaves and the way it transmits and causes disease every day."

Researchers are also still learning more about why the virus strikes some people more harshly than others, as was the case with Stephanopoulos and Wentworth, according to Ashton.

"One of the theories is it has to do is how much of the actual viral particles someone gets exposed to, that may be one part of it," she said. "Your immune reaction to the virus may be another part. Where the virus actually lodges … may have something to do with it."

"In terms of the symptoms, it’s important for people to understand that when we say about 80% of the cases are mild, that doesn’t mean pleasant," Ashton added. "Ali's [Wentworth] case clinically was defined as mild because she didn’t need hospitalization but it certainly wasn’t pleasant."

Ashton also said researchers are "still learning" whether people who have had COVID-19 then develop antibodies to the virus.

"Every time we’re exposed to a virus, yes, we develop some immune reaction or protection but when that occurs, how strong it is, how long that will last, all unknowns," she said. "And remember there are slightly different strains of this virus, just like any other coronavirus or cold virus, and you could be exposed to a different one and get sick."

Ashton noted at the time of Wentworth's diagnosis that Stephanopoulos and his family were practicing the "home version of social distancing" that must be done when a person in the household tests positive for COVID-19.

"The key is really isolation and you guys are doing it right," she said earlier this month. "We have to remember that everyone in the household who has been exposed should presume that they are infected until they’re out of a 14-day window of observation."

"Isolating, keeping them out of the kitchen or wiping down surfaces in any common areas is really, really important," Ashton explained. "That’s the key."

What to know about coronavirus:

• How it started and how to protect yourself: coronavirus explained
• What to do if you have symptoms: coronavirus symptoms
• Tracking the spread in the US and Worldwide: coronavirus map