A culture of mask wearing. Japanese people (like many in east Asia) have long worn masks at the drop of a hat, so were early adopters in the case of COVID.
A culture of not touching. Bowing is much more the thing than handshakes and hugs, so less chance for transmission.
An early and strong emphasis on the importance of good ventilation and good hygiene. Seems very plausible to me, though perhaps not a sufficient explanation. The necessity of not living in crowded conditions would probably fall under this heading.
Body shape. Japanese people tend to be thin, and problems such as high blood pressure (a risk factor for COVID) are less prevalent.
Diet. Could it be something in the food that gives resistance to some but not others?
Genetic differences. Could there be some form of genetic resistance shared by east Asians and Africans but not Europeans? I discussed this with Haawa, but it seems unlikely, given that black people in Britain seem to be more vulnerable to the disease, not less.
Climactic differences. Given the diversity of the regions in which the virus has spread, and also of those in which it has not, this early contender has recently lost favour.
The Japanese have a higher "mindo". This suggestion, which I include for the sake of completeness, was recently thrown out by a Japanese politician, Taro Aso, who has a habit of saying embarrassingly semi-racist things. Mindo (民度) essentially means "class of person."
I think I've probably left a few out, so may add to this list as other things occur to me.
Meanwhile, here's another big mystery: why is the UK's death rate so large? According to official statistics, in this country about 15% of people who test positive for COVID go on to die of it. This is far higher than, say, the USA, which has the most cases and the most deaths but where the death rate figure is more like 3 or 4% (something Trump was boasting about the other day, although of course there are many countries with better rates than that).
a) the UK is just really really bad at keeping COVID patients alive. This seems unlikely, when the medical care here is on a par with that of most Western nations.
b) the UK is home to a particularly deadly strain of the virus. Odd that no one has mentioned it, if so.
c) far more people are catching the virus than appear in the figures, and the real death rate is thus artificially depressed. This seems plausible at first glance. Testing in the UK is now at a very respectable level, but it may be that the lack of it in the early days of the pandemic is still skewing the total figures. However, even if you just take deaths vs. new cases for the last seven reported days, you still get a death rate of over 11%.
If you have other suggestions, I'd be very interested to know them.