Next summer, tens of thousands of sports fans will enter Japan to attend the Olympic games — but along with paraphernalia from their home countries, the tourists may be carrying lethal pathogens onto Japanese soil.

To mitigate the risk of potential outbreaks, Japan imported the Ebola virus and four other deadly pathogens in September in order to prepare diagnostic tests, according to news reports.

The pathogens represent the most dangerous viruses ever allowed to enter Japan, according to a report in Nature . All rated "biosafety-level-4" (BSL-4), the viruses must be held in a special containment facility where researchers follow strict safety protocols. The only Japanese facility that meets these requirements — the Japanese health ministry's National Institute of Infectious Diseases — is in Musashimurayama, about 19 miles (30 kilometers) west of Tokyo..

Besides Ebola , the facility contains four other related viruses: The Marburg and Lassa viruses and viruses that cause South American hemorrhagic fever and Crimean-Congo hemorrhagic fever, according to the Nature report. The live samples will be used to validate diagnostic tests that determine whether a person with one of the viruses is still infectious. The test assesses whether a person is generating antibodies to fight the virus, which would suggest they are in recovery, Masayuki Saijo, director of the NIID department responsible for hemorrhagic-fever viruses, told Nature.

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The BSL-4 lab requires researchers to wear full-body, air-supplied, pressurized suits; change their clothing before entering; shower upon exiting; and decontaminate all materials before exiting, according to the Centers for Disease Control and Prevention . The lab itself must be housed in a separate building or an isolated wing within a larger facility must be equipped with dedicated supply exhaust air systems, vacuum lines, as well as decontamination systems.

The NIID lab stands as one of the few BSL-4 facilities in Asia, while the U.S. and Europe each have about a dozen such labs in operation or under construction, according to Nature. "This is a landmark time, a landmark event" for NIID, Saijo said in an announcement on Sept. 27, according to The Japan Times .

"We have come to a good level of understanding on the matter. It is a major stride toward protection" against the potential threats of the viruses as the country prepares to welcome spectators of the sporting event from all over the world, Takumi Nemoto, health, labor and welfare minister of Japan, told Kyodo News .

However, Japanese residents living near the facility harbor serious concerns.

The NIID announced its intention to import the viruses in November at a public hearing, where local residents protested the plan, according to The Japan Times. "It is nonsense for the government to tell us to accept the plan because of the Olympics," a representative of the Raizuka residents' association, who lives near the storage facility of NIID's Murayama Branch Laboratories, told The Asahi Shimbun . "We are worried and cannot accept it."

Although the NIID facility was built to handle BSL-4 pathogens back in 1981, resistance from locals prevented the institute from bringing viruses on-site, according to Nature. In 2015, the health ministry and the mayor of Musashimurayama finally cleared the lab to operate as a BSL-4 facility, potentially in response to the Ebola outbreak in West Africa, according to an earlier Nature report .

But until this year, no BSL-4 pathogens had been imported. Until now, Japanese researchers had to apply for access to BSL-4 labs overseas, which are in high demand, virologist Ayato Takada at Hokkaido University in Sapporo, Japan, told Nature. Experts told Nature that importing the viruses should allow researchers to prepare for possible outbreaks during the Olympics or after, and study related diseases carried by animals.

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"A report of an Ebola virus infection during the Olympics could have devastating consequences if the emergency responses were not professional," microbiologist Elke Mühlberger of Boston University told Nature.

Other scientists think the global proliferation of BSL-4 labs could place humanity at greater risk of bioterror attacks.

Storing dangerous viruses, even in a highly secure lab, increases the risk of an accidental or a deliberate release, Richard Ebright, a molecular biologist and biosecurity specialist at Rutgers University in Piscataway, New Jersey, told Nature. Japan could prep for a potential Olympic outbreak without first importing the pathogens in question, he argued, and may be one of several governments "stockpil[ing] deadly agents to deter bioattacks from similarly equipped adversaries."