More than 4,100 people were aboard the Carnival’s Crown Princess Cruise line when a highly contagious virus spread across the ship, infecting 172 people by the time they docked in California. The Centers for Disease Control and Prevention (CDC) announced on Sunday passengers had fallen ill with norovirus during the 28-day cruise.

"As it is the cold and flu season, when the stomach flu circulates on land, we encourage all of our guests to be diligent in following the widely accepted practices of frequent hand washing with soap and water and the use of hand sanitizers," company spokeswoman Susan Lomax said. This past April, 129 people on the same ship contracted norovirus during a seven-day cruise set off the California coastline. Lomax said the ship underwent a thorough disinfecting process as soon as the ship docked.

Approximately 20 million people catch the norovirus each year from contact with infected people or by eating contaminated food. Symptoms include vomiting, diarrhea, fear, and body aches, according to the CDC. What’s alarming is norovirus is the leading cause of disease breakouts from food in the U.S., and 70 percent of cases are caused by infected food workers. Vomiting and diarrhea are the most common and frequent symptoms, but some may need hospitalization and in extreme cases may die. Norovirus outbreaks aren’t unique to cruise ships and can spread anywhere, but the close living quarters don’t help contain the illness.

The CDC recommends food service workers practice proper hand washing and avoid touching ready-to-eat foods, such as raw fruits and vegetables with their bare hands. Kitchen managers should be certified in training the service with the correct food safety practices. One in five food service workers have reported they were so sick they were vomiting and had diarrhea. It’s the fear of job loss or leaving a coworker short-staffed that sends sick workers to their restaurant or kitchen, according to the CDC.

Out of all the norovirus outbreaks, 54 percent were caused by infected food workers who touched ready-to-eat foods with their bare hands. The virus only needs the very small amount of 18 viral particles to spread from person-to-person or food-to-person. The amount of virus particles that fit in the head of a pin would be enough to infect more than 1,000 people. Norovirus is actually pretty difficult to kill because it resists common disinfectants and hand sanitizers. Unless it’s cleaned with heavy duty cleaning supplies, it will remain on countertops and serving utensils for up to two weeks. It can even stay alive through extreme colds and extreme high temperatures.

Currently, the ship is undergoing a deep cleaning to ensure their upcoming voyage to the Mexican Riviera goes smoothly without worry of traveling with the contagious germ. The CDC is scheduled to board the ship to investigate the outbreak and how the cruise line plans on handling their health and safety procedures in the future.