Many Americans will be exposed to COVID-19 over the next year or so with many people in the U.S. getting sick, a top CDC official said Monday, recommending that people over 60 and anyone with chronic medical conditions buckle down for a lengthy stay home.

"This virus is capable of spreading easily and sustainably from person to person ... and there's essentially no immunity against this virus in the population," Dr. Nancy Messonnier, director of the CDC's National Center for Immunization and Respiratory Diseases, told reporters on a conference call, citing World Health Organization data that studied more than 70,000 cases in China.

"It's fair to say that, as the trajectory of the outbreak continues, many people in the United States will at some point in time, either this year or next, be exposed to this virus and there's a good chance many will become sick," she said. Most people won't develop serious symptoms, but 15% to 20% of the people who are exposed to the virus get severely sick, she said.

Of the 70,000 cases WHO scientists looked at, only about 2% were in people younger than 19. The odds of developing COVID-19 increase with age, starting at age 60. It's especially lethal for people over 80.

"This seems to be a disease that affects adults and most seriously older adults," she said. "Starting at age 60, there is an increasing risk of disease and the risk increases with age."