Taking a cruise evokes sun, fun, fine dining, relaxation and romance -- the ingredients of a perfect vacation.
But also lurking on these ships in places you can see -- and places you cannot -- may be a cast of stowaways that could potentially ruin your holiday.
Most notable among them is the norovirus, a nasty bug that can cause a couple of days worth of severe nausea, vomiting, diarrhea, fatigue and headaches. The norovirus can spread easily when large groups of people are gathered together in close proximity, places like restaurants, schools, nursing homes, hotels -- and cruise ships.
Last month alone, there were seven norovirus outbreaks on cruise ships.
ABC News' "Primetime Live" sent teams of producers equipped with test tubes, gloves and special lights on four different cruise ships.
They did not find any norovirus. But they did find other things that were disgusting, though not actually dangerous to your health -- dried semen and urine on blankets, furniture and walls in each of the cabins in which they stayed.
For example, on one cruise ship headed to the Bahamas, an ultraviolet lamp exposed dried urine by the door handle and on the vanity stool. Dried semen was on the carpet and on the blanket. The cabin cost nearly $2,000 a week.
Dr. Aaron Margolin, a leading microbiologist and viral expert at the University of New Hampshire who consults for the government, told ABC News he thinks such stains are not uncommon on cruise ships or in hotels and motels. "I think what's new is that we have new ways of detecting it so that we now know that it really exists," he said.
In fact, "Primetime Live" found similar results two years ago when it swabbed for stains in hotel rooms in establishments ranging from one-star to five-star. All 20 rooms had urine or semen stains on bedding, furniture or walls -- in one room, on the Bible.
"I think when an individual goes to a hotel, motel, a cruise ship, they expect that the area is sanitarily disinfected. I think what they have to realize is that is not the case," Margolin said.
At least not always. A sweep of a black light in a cabin on a second ship, for example, revealed dried urine on the walls, and dried semen on the door of the fridge and on the dresser.
Margolin was at a loss for explaining how the stains got where they were. "I can only use my imagination, and maybe that's what these people are doing also," he said.
On other cruise ships, "Primetime Live" producers found dried urine on a cabin table and dried urine or semen stains on the bedspread, the night table, the phone, the couch, the coffee table, the TV, the closet door and even a life vest.
All told, urine or semen stains were found in all nine cabins of the four ships tested -- 57 out of 118 samples -- including on an ice tray.
But while such stains may well turn your stomach, experts say they are very unlikely to actually make you sick.
"I don't want to say it's impossible but it's not likely," Margolin said. "But I think it more demonstrates the unsanitary quality of the room and then what you have is a greater probability that other micro-organisms can also be transmitted."
Beware the Norovirus
There is a bigger issue about unsanitary conditions: they can harbor things that may make you sick, like norovirus.
More than 2,000 people came down with the virus on eight cruise lines last year. Other cruise lines have had outbreaks in previous years. The Centers for Disease Control and Prevention maintains an up-to-date listing of norovirus and other outbreaks on cruise ships, identified by cruise line and ship.
ABC News producers tested four of those cruise lines that had outbreaks last year. Samples were sent to Margolin's lab, and no norovirus was found. But the doctor said he wasn't surprised because the size of the samples compared with the overall ship surface area was small.
"Norovirus is a hearty viral bug and can end up in food, swimming pools and on numerous surfaces that people can touch," he warned. "It can spread easily in places where people are brought together in close proximity, including cruise ships."
Cathy Wos and her husband were among more than 200 people who got ill on a cruise to the Caribbean just last month on Holland America's Veendam.
"Really, really sick," Wos said. "We're talking about you're in the bathroom and I'm puking in the tub. I mean, you're going both ends."
They were stuck in their room sick for four days on their $4,600 vacation, missing half of the sightseeing ports.
"Drinking water and eating Jell-O is not what I was contemplating doing on a cruise," she said.
The cruise line industry points out that far more people get norovirus on land in places like restaurants, schools, nursing homes and hotels than on cruise ships.
Cruise line representatives told ABC News, "according to the U.S. Centers for Disease Control and Prevention, cruise ships are held to the highest sanitation standards of any public facilities in the world." And they added they have "always considered the safety and comfort of passengers our highest priority."
Still, Margolin warns, "Whether you're taking a cruise, whether you're going out to dinner tonight, whether you're staying at a hotel, we should still enjoy ourselves but realize there are calculated risks that one is taking."
To reduce the risk, everyone agrees hand washing is critical -- but it's not always easy. On one ship on which the "Primetime Live" producers traveled, one heavily used public bathroom had a soap dispenser that did not work properly the entire trip.