The Centers for Disease Control and Prevention announced that 400 people aboard two cruises in the Caribbean have fallen ill. The U.S. health officials added that it is conducting lab tests to identify the cause of the disease.

The affected cruises are Cunard's Queen Mary 2, where 194 of 2,613 (7.42 percent) passengers have gotten ill and Princess Cruises' Emerald Princess, where 189 of 3235 (5.84 percent) passengers have contracted the disease. According to CDC guidelines, Cruise ships participating in the Vessel Sanitation Program are required to notify the agency if the gastrointestinal illness count onboard exceeds two percent of the total number of passengers or crew.

"Outbreaks occur throughout the year although there is a seasonal pattern of increased activity during the winter months," Jaret Ames, of the CDC's Vessel Sanitation Program said, AFP reported.

He added that in closed spaces like a ship, or hospitals, infection can spread faster. Health officials say that the most likely cause of the infection could be norovirus.

According to Vessel Sanitation Program, norovirus is a very contagious virus, and contaminated food, water and surfaces can cause infection. The virus causes inflammation of stomach, intestine or both. The swelling leads to stomach pain, nausea and diarrhea.

"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," Ames explained, AFP reported.

Cunrad said that there are 19 passengers onboard Queen Mary 2 who have "active symptoms" of the disease. In a statement to ABC News, the cruise line said, "there has been an incidence of a mild gastrointestinal illness among the passengers on Queen Mary 2. This illness is suspected to be Norovirus, which is highly contagious and typically transmitted from person to person."