It started with just one infected passenger.
Then there were a few more. Days later, that quickly jumped to dozens.
And now there are 219 cases of coronavirus cases linked to the Diamond Princess.
Forty-four people were diagnosed on Friday, even though the entire ship has been in a mandatory 14-day quarantine since February 4.
With the number of cases of coronavirus climbing, but all passengers still in quarantine, just how is this disease spreading onboard?
The new cases aren't necessarily new infections
Satoshi Hori, a Professor of Infection Control at Tokyo's Juntendo University, does not think the new cases mean the quarantine is failing.
"I believe the tests are only being done in batches and that's why the new cases are being reported this way," he said.
Australia's chief medical officer said it seemed like everyone was infected before the quarantine was put in place.
"But if further cases continue to come, you'd have to wonder about the quarantine," Professor Brendan Murphy said.
Australia is sending a public health expert to Yokohama in the next few days to check out the situation.
Ian Mackay, a leading virologist from the Australian Infectious Diseases Institute, questioned the wisdom of putting the ship on lockdown in the first place.
"It would have been a much better idea to have these people taken off the boat in the beginning, and put into proper quarantine and isolation environments," Associate Professor Mackay said.
"It would have been better for the people — more humane certainly — rather than keeping them on a boat where perhaps quarantine and isolation hasn't been done to the best of its usefulness."
Medical experts who have been on board said the protective measures inside the ship were largely good.
Professor Shigeru Sakurai led a team of infectious diseases experts that surveyed the ship.
He said the crew was following the manual and taking infection prevention measures, but there was some room for improvement.
"There were insufficient parts such as disinfecting hands and fingers among the crew so we gave some guidance," he said.
"There was a lack of awareness that the virus on hands could spread the infection."
Many of the crew are foreigners so Professor Sakurai's Japanese team showed illustrations and videos to the crew to teach disinfection methods.
Why don't authorities just test everyone on board?
In short, they just do not have the capacity to do so.
Government facilities can only test around 300 samples per day.
With 3,600 people still on board the ship, that would be an extraordinary logistical effort.
Japan's Prime Minister Shinzo Abe said the Government would increase the capability to 1,000 per day by allowing private facilities to test for the virus.
The goal is to get that done by February 18, the day before many of the passengers are supposed to be allowed off the ship.
More than 200 passengers and crew have now tested positive, including one quarantine officer who was on board taking temperatures and collecting questionnaires.
How did a quarantine officer get infected?
Japan's Health Ministry said the quarantine official had thoroughly disinfected himself each time, but was not wearing a full-body protective suit.
"It's a relatively low-risk task," Professor Hori said.
"If the officer had been dealing with a large number of people, there's a chance that the small amount accumulated and as a result, the person was exposed to a massive amount of the virus."
The quarantine official may have touched the surface of his protective gear when removing it, and then touched his face.
"Such an accident can happen with infectious diseases, and it's hard to believe that the virus has mutated."
Associate Professor Ian Mackay has dismissed a theory that the disease might be spreading through the ship's ventilation system.
"Even if it was airborne, the virus particles travelling through convoluted air ducting into other rooms, it can happen, but I think it would be a minor route of transmission," he said.
How are the crew coping?
While passengers are trying to pass the quarantine in their cabins with movies and card games, it's a different story below deck.
There are more than 1,000 crew members on board the Diamond Princess, and so far 10 have been infected.
They have been given masks and gloves to help prevent the spread of infection, and thermometers to monitor their temperatures.
But they have to prepare and deliver meals to the passengers, and clean up the utensils and cutlery.
Crew also must share bathrooms and cabins with their colleagues.
Binay Kumar Sarkar, who works in the ship's kitchen, has posted videos on social media imploring the Indian Government to rescue him.
"Day by day it's spreading out rapidly and risking everyone's life," he said on Facebook.
In a video with four other Indian crew members, Mr Sakar said none of them had been tested for coronavirus.
"We are in a state of panic. We want to appeal to the Indian Government to rescue us from here."
On its website, Princess Cruises said the "safety, security and wellbeing of all guests and crew is our absolute priority".
So how much longer does everyone on board have to wait?
The official quarantine end date is February 19, but there are a few caveats.
Infected patients are all being taken off the ship and to specialist infection wards.
The wards are negative pressure rooms with doctors and nurses in protective gear.
An Australian woman in one of these hospitals described it as "total isolation for 10 days at least, in a box with nothing to do".
"This is going to be a real test of spirit and patience," she said.
"I just keep sticking my head at the window to see if I can see down the corridor and see anything. But you're not supposed to do that I don't think."
People who have been in close contact with infected cases will have their quarantine date reset to the last day they met.
The Government is now planning to remove passengers who are over 80, have a serious illness or are in cabins without windows or windows that open.
If they test negative for the virus, they will go to a specially prepared government facility.
Everyone else on board is counting down to February 19, and hoping they stay healthy until that point.