The US Centers for Disease Control and Prevention says "older adults" and people with serious chronic medical conditions "are at higher risk of getting very sick from this illness."

Anyone over 60 and those with underlying health problems should try to avoid places with large crowds — such as movie theaters, busy malls and even religious services, top infectious disease experts say.

"This ought to be top of mind for people over 60, and those with underlying health problems," said Dr. William Schaffner, a Vanderbilt University professor and longtime CDC adviser.

"The single most important thing you can do to avoid the virus is reduce your face to face contact with people."

But why is age 60 often used as a threshold for those who need to be extra cautious?

"We now know more about who is at risk," US Surgeon General Jerome Adams said.

"(The) average age of death for people from coronavirus is 80. Average age of people who need medical attention is age 60."