A massive influx of international tourists will arrive in Japan next July for the Tokyo Olympics. Most will be unaware another foreign visitor was specifically imported in preparation for their arrival: Ebola.

Strains of the infectious virus along with four other dangerous pathogens were brought into the country last month by the Japanese government, so that scientists can study them and research possible countermeasures in the event of an outbreak sparked by the Tokyo 2020 tourist influx.

In addition to Ebola, the Japanese health ministry's National Institute of Infectious Diseases (NIID) has imported the pathogens responsible for four other kinds of viral haemorrhagic fever, including Crimean-Congo haemorrhagic fever, South American haemorrhagic fever, Marburg disease, and Lassa fever.

All of these are highly dangerous viral agents that have not been introduced into Japan before, and represent the first time that pathogens belonging to the most dangerous class of biological agent (biosafety level 4) have been admitted into NIID facilities.

For the researchers who will study the viruses, it's an unprecedented chance to investigate pathogens that are necessarily very difficult to come by, and to demonstrate the capabilities of Japanese scientists.

"This is a landmark time, a landmark event" NIID departmental director Masayuki Saijo told Nature.

The pathogens were transported to an NIID facility in the district of Musashimurayama on the western side of Tokyo, but local views on the new arrivals are understandably mixed.

In July, when the virus importation became official, Japanese health minister Takumi Nemoto announced he had the approval of local Musashimurayama authorities.

"We have come to a good level of understanding on the matter," Nemoto said. "It is a major stride toward protection."

But while Musashimurayama's mayor, Masaru Fujino, may have approved the NIID's new viral research program, not all residents are happy about having Ebola on their doorstep, fearing a potential outbreak if containment protocols somehow fail.

"It is nonsense for the government to tell us to accept the plan because of the Olympics," a residents' representative explained to The Asahi Shimbun last November while the plan was still under discussion.

"We are worried and cannot accept it."

Despite those concerns, the controversial plan is now underway, with NIID authorities insisting the risks of not studying the pathogens outweighs the risks of failing to control them on Japanese soil, especially as Tokyo 2020 fast approaches.

"Even though there have been no outbreaks of these diseases in Japan, Tokyo will host the Olympics in 2020 and there is an increased risk of one of these viruses being brought into the country," Saijo told Japanese media in July.