The arrival and settlement of Europeans in the Americas resulted in what is known as the Columbian exchange. During this period European settlers brought many different technologies, animals, plants, and lifestyles with them, some of which benefited the indigenous peoples. Europeans also took plants and goods back to the Old World. Potatoes and tomatoes from the Americas became integral to European and Asian cuisines, for instance.[3]
Colonial epidemic disease in Hawai‘i has greatly threatened the Native Hawaiian population since its introduction to the islands over a hundred years ago. Beginning with the first colonizers led by Captain Cook that arrived in the islands in 1778,[1] all the way up until today, foreign disease has been present in Native Hawaiians. Because Hawai’i was so secluded and contained a population of only the Hawaiian people, the islands were considered a “virgin population."[2] This meant that once foreigners arrived, the Native Hawaiian population was decimated by these illnesses while Europeans remained healthy. Some of these diseases included gonorrhea, syphilis, influenza, cholera, tuberculosis, the mumps, measles, smallpox, and leprosy (which lead to the creation of a leper colony on Moloka‘i in the mid 1800s).[2][3] While each disease brought a different outcome, they all contributed to the reduction of the Native Hawaiian population as they collectively caused more than 100,000 deaths.[4]
So what happened to Japan when Europeans started showing up there? The Portugese showed up in the 16th century, and the Dutch around the same time or soon after - and the US really pried the door open in the mid 1800s. But I don’t hear stories of terrible epidemics following their arrival. What happened (or didn’t happen)?
I’m not a historian, but I think that the Japanese as old world people like the Europeans had about the same exposure and immunity to the diseases the Europeans had.
Japan was never actually isolated though. The isolation lasted from 1603 until Perry forced the issue. But during the isolation there was still contact with mainland Asia.
Japan would have been exposed to most of those diseases already, indirectly, through Asia. And the reverse: I didn’t read any stories of visitors to Japan getting sick either.
(I suspect that there were plagues, but nothing like the onslaught of multiple diseases which hit American natives.)
Prior to the “Columbian Exchange” there was very limited contact between American natives and people from the outside. There were some Viking contact, but that didn’t last long, and these natives were far away from the earliest Spanish colonists. (I am not counting Greenland here, by the way.) Then all of a sudden the natives faced sustained contact without previous exposure.
Japan had had extensive contact with China for thousands of years, and China had extensive contact with the rest of the world. The European explorers who arrived in the 16 - 17th centuries would not have brought anything new, communicable disease wise.
Another point is that a lot of the Euro-plague diseases hopped back and forth between domestic animals and humans and wildlife like birds (and bats). Humans living in close proximity to pigs were particularly vulnerable. Japan was not isolated enough to avoid these, and had the same domestic animals as rest of Eurasia/Africa land mass. Also, they had the centralized political organization where diseases could travel with people commonly from one concentrated population center to another.
I would probably hedge a bit on what everyone else has said. While there would have been a connection between Japan and Europe, through the Silk Road and through European shipping in the time leading up the Europeans discovering Japan, we should probably expect that the ability for diseases to spread was lower than in modern times and so there probably were somewhat regular plague outbursts through Eastern Asia (and, likewise, in Europe).
For example, smallpox seems like it may have appeared around the 3rd century BC in the Middle East. From there, it gradually was able to spread to Europe (probably hitting around 170 AD), moved East into India around 400 AD, and may have only properly found its way to Japan by around 730 AD, killing about 1/3rd of the population of Japan:
So it’s not really that there wasn’t the potential for diseases to develop for long periods in Western Eurasia and then, suddenly, turn up in the far East some centuries later, with devastating results. But, it wasn’t so so complete as it was with the New World. Rather than Japan getting hit with one, giant wave of all varieties of pestilence, they were able to get them more one-at-a-time, with slightly less horrific results.
And in some cases, the Japanese may have done better than other places due to their fondness for bathing and cleanliness. Good hygiene helps to mitigate some diseases. They may have been able to skip out on some plagues that really gave a bashing to Europe and the Americas.