The NBA held its playoffs without a single COVID case. Why can’t the Tokyo Olympics do the same?

Athletes from the world over are converging in Japan ahead of the delayed Tokyo 2020 Olympics, which open on Friday. The Games were postponed last year owing to the pandemic, but, 12 months later, Tokyo is still in the grips of a COVID-19 outbreak, and 67 people associated with the Olympics have tested positive for COVID-19 since July 1.

International Olympic Committee (IOC) president Thomas Bach argued that the positive cases demonstrated how systems designed to catch COVID cases were effective.

“I think this shows that the measures are not only in place but that they are working and that they are enforced,” Bach said on Saturday.

At the time, there were only 44 Olympic-related infections. The number is growing.

Subscribe to Eastworld for weekly insight on what’s dominating business in Asia, delivered free to your inbox.

On Monday, U.S. gymnastics alternate Kara Eaker and Czech volleyball player Ondřej Perušič became the latest athletes to test positive for the virus. A day earlier, two South African soccer players tested positive in the Olympic Village, too.

Since the pandemic began, other sporting events have managed to convene without competitors catching the virus—even before COVID-19 vaccines were available. The NBA famously kept players in an isolated “bubble” for 172 games of the 2020 season, without a single case of COVID-19.

But Tokyo 2020 is a different challenge. Some 60,000 athletes, staff, and media are traveling to and through Japan in order to attend the Olympics, and the rising case numbers related to the Tokyo 2020 Games reveal just how tricky keeping all those attendees in a bubble will be.

What are the protocols?

In April, Japan’s National Olympic Committee (NOC) released a series of “playbooks” for delegates attending the Tokyo 2020 Summer Games, outlining what safety precautions the attendees would be required to take before and during the games.

The playbook for athletes instructs participants to monitor their temperatures for two weeks prior to arriving at the Olympic Village, to take a test two days before arrival, and again before they enter the village. Tokyo doesn’t require athletes to be vaccinated before arriving at the village, but, according to the IOC, around 80% of competitors at the Olympics are.

The rules also prohibit athletes from entering the village earlier than five days before their events and mandate that athletes leave the village 48 hours after their final performance. During their stay, athletes are restricted to traveling between areas of the village; they cannot enter Tokyo proper.

Teams are required to book tables at the Olympic Village dining room, to minimize mixing, and all athletes are tested daily. If a positive case is detected, all close contacts are forced to self-isolate in their rooms while undergoing testing.

Six members of the Great Britain team are currently under isolation, after coming into close contact with a person who tested positive upon arrival at the village last Friday. The original carrier was not a member of Team GB.

What are the weak spots?

The key weakness in Tokyo 2020’s COVID-19 screening policies is that many of the athletes arriving at the village will have traveled by commercial, or public, transport. Unlike during the NBA bubble, where basketball stars were flown to their bubble in Walt Disney World by private jet, athletes are arriving in Japan by commercial flight alongside ordinary travelers.

Last week, two passengers sharing separate flights with Kenyan and South African Olympic rugby teams tested positive for COVID-19 upon arrival in Japan. The cases threatened to send both teams to a mandatory 14-day quarantine, so the Tokyo NOC changed its rules to be more lenient on when athletes should self-isolate.

The rule change has introduced more room for error. Because the virus can incubate for up to two weeks, attendees who contract the virus while traveling to the village might not test positive until days later, during which time they might have mingled with hundreds of other people—either other athletes in the communal gym spaces and canteen, or staff manning the facilities at the village.

The Olympics are also taking place in several stadiums and sites across the country. Ushering athletes from their accommodations in the Olympic Village to their events at the Olympic stadiums will present more opportunity for transmission. The NBA, by comparison, held all of its matches at venues inside Disney World.

Athletes are expected to travel only between the village and their relevant event venues, without straying into Tokyo’s general city, and private shuttle buses will ferry athletes to and from the venues. The NOC said in June that the locations of athletes and others will be tracked by a smartphone app, too, but that the system will only be used to conduct contact tracing, not to monitor movement in real time.

Subscribe to Fortune Daily to get essential business stories straight to your inbox each morning.