Mumps Tests Sideline NHL's Sidney Crosby

Mumps 'Rampant' in National Hockey League

Crosby "has not been feeling well" and will miss the next two days while he undergoes testing for the viral illness, Penguins general manager Jim Rutherford said.

"There is no indication at this time that this is the mumps, but we are going to hold him out as a precaution," Rutherford said. "We'll have additional test results in a few days."

Although mumps was once common in the United States, most Americans are immunized against it via the MMR vaccine while toddlers. But the vaccine's protection weakens over time.

Dr. William Schaffner, who has not treated these hockey players but is chairman of preventive medicine at Vanderbilt University Medical Center in Nashville, Tennessee, said this outbreak reminds him of mumps outbreaks that occur at small colleges because those affected are young adults, including some from foreign countries that don't have the U.S.'s "comprehensive" mumps vaccination program. And the players are in close contact.

"What you have in a traveling team that plays together and practices together -- sit on airplanes together -- is prolonged close contact," Schaffner said. "If somebody is sick they need to stop playing, traveling. This virus can spread even before you become sick."

ABC News's Aaron Katersky and Troy McMullen contributed to this report.