More than 100 people have reportedly gotten sick while on board a cruise ship headed for Galveston.

Princess Cruise Lines says 102 people on board the Crown Princess have come down with the norovirus, a painful gastrointestinal illness.

The cruise line believes it was brought onboard by an ill passenger.

Dr. Brent Vasut at the University of Texas Medical Branch at Galveston said it doesn't take much exposure to catch.

"It usually starts out with a fever, 50 percent develop acute vomiting, which can be pretty severe," said Dr. Vasut. "Almost all of them have watery diarrhea."

Princess Cruise Lines said the number of ill passengers has returned to normal levels after a thorough shipboard cleaning. The line said the ship will undergo a comprehensive disinfection when it arrives in Galveston on Saturday.