Japan is facing the threat of a wave of coronavirus infections hitting hospitals even as it grapples with yet another cruise ship cluster.

The Keio University Hospital in Tokyo tested 67 non-coronavirus patients before their admission or surgery this month, and found that four were positive despite showing no symptoms.

The hospital said in a statement that it will ramp up cautionary measures even for non-Covid-19 patients, adding that the data “reflects the reality of community transmissions”.

Japan already has multiple hospital mega-clusters including the Hokkaido Cancer Centre in Sapporo, where 57 people have tested positive since April 16.

A spokesman told public broadcaster NHK: “Since the route of infection is unclear, tests are being expedited for the more than 600 cancer patients and staff on the pretext that all of them might already have been infected.”

Other shocking mega-clusters in Tokyo include the Eiju General Hospital, with at least 24 deaths among 187 cases, and the Nakano Egota Hospital with at least 92 cases.

This indicates a clear risk of an asymptomatic Covid-19 carrier seeking treatment for another problem, but inadvertently spreading the coronavirus in a hospital.

How Japan acts to prevent the formation of clusters and to contain further outbreaks have been under scrutiny, ever since the Diamond Princess cruise ship off Yokohama recorded 712 cases and 13 deaths. There were 3,711 people registered on the ship’s manifest.

Now, two months later, the Costa Atalanta cruise ship docked off Nagasaki for repairs has become another hotbed for cases, with 148 reported so far among the 623 people on board.

Japan has been hesitant to test more widely over fears that doing so would trigger a collapse of its medical system.

Covid-19 has exposed a dire shortage of masks, ventilators and personal protective equipment for healthcare workers, as well as beds for patients. At least four people have died of Covid-19 at home while awaiting treatment.

Little has also been done in sniffing out mild or asymptomatic cases, many of whom, no doubt, are inadvertently spreading the virus without being aware of it.

Japanese Prime Minister Shinzo Abe on Saturday (April 25) repeated his appeal for people to stay home to protect themselves and others.

“We must pay utmost attention and recognise that we all not only have the risk of being infected, but at the same time have the risk of spreading the virus onto others.”

Covid-19 already has threatened to hit close to home for Mr Abe, whose point man for coronavirus strategy, economy minister Yasutoshi Nishimura, isolated himself on Saturday after learning that he had been in close contact with an infected Cabinet Office official. They had gone to the University of Tokyo Hospital on a work visit last Sunday.

Japan is under a state of emergency until May 6 and the public has largely heeded calls to stay at home and avoid travel, though residential areas are more crowded.

The “strong appeals” for people to exercise restraint do not come with criminal penalties – freedom is a right guaranteed in the Constitution – but local governments can exert social pressure by “naming and shaming” businesses into closing, as Osaka did last Friday for six pachinko (slot machine) parlours.

Only two of the six closed.

As of 8pm on Saturday, Japan’s tally was 13,191 coronavirus cases and 351 deaths.