Authorities in Tokyo have confirmed over 200 cases to date, and 41 of them came in a single day.
Dr. Norio Ohmagari, the director of Disease Control and Prevention Center, says it isn't just the spike in number that's worrying. It's also that authorities can't trace the route of infection for at least 10 of the new cases.
If the routes become unclear, there is a greater risk of a hidden growth of infections. And experts warn that could lead to an explosive surge in the number of cases.
Tokyo traditionally gets an influx of people from around the country at this time of year. Young people come to the capital to begin new jobs or attend new schools.
Governor Koike says she's concerned that young people who haven't developed symptoms don't know they are infected. She's asking schools to delay the start of the new year.
Koike said Tokyo is "now at a critical juncture" and appealed to residents to grasp the gravity of the situation and work from home as much as possible. She also called on people to refrain from going out in the evening and this weekend.
The governor has already said the capital may have to take extreme measures. On Monday she suggested a lockdown was possible if the virus spreads further.
Dr. Kazuhiro Tateda, an expert in infectious diseases, says there could be a surge in cases in the next couple weeks, and the situation in Tokyo may become similar to that of Italy and France. He said Japan's capital is in an extremely dangerous situation.