The CDC said it was still conducting lab tests to determine the pathogen, but it indicated it might be Norovirus, which is highly contagious and typically transmitted from person to person.
Both ships, Cunard's Queen Mary 2 and Princess Cruises' Emerald Princess, reported the outbreak to the Centers for Disease Control, following guidelines that come into play when more than two percent of the passengers and crew are hit.
A similar outbreak earlier in the month hit the P&IO Oriana liner on a 10-night Baltic cruise, infecting about 300 of the 1,843 travelers aboard.
Such outbreaks are not uncommon, said Jaret Ames, of the CDC's Vessel Sanitation Program.
"Outbreaks occur throughout the year although there is a seasonal pattern of increased activity during the winter months," he said.
He said in an enclosed area like a ship -- as well as in places like hospitals and nursing homes -- the illness can spread quickly from person to person.
"Those with symptoms many times circulate among those who are healthy; if they have an event of public vomiting or they don't wash their hands well before touching surfaces, certain illnesses such as norovirus are spread easily from one ill person to one or more well persons, and that illness spreads along that pattern," he explained.
The public health agency said both liners had taken steps to stem the outbreak, including increasing cleaning and disinfection procedures, and keeping passengers informed.
On the Queen Mary 2, which embarked from New York on December 22 for a 10-day cruise, 194 passengers and 11 crew members of the more than 3,800 people on board have reported being ill, the CDC reported on its web site.
And on the Emerald Princess, owned by Princess Cruises, which returned to Fort Lauderdale on December 27, 189 passengers and 31 crew members of the more than 4,400 people on board fell sick, it said.