By Dr Irina Metzler, FRHistS One of the puzzles in the Covid story is how different the effect of SARS-CoV-2 can be from person to person. If we accept the notion of ‘asymptomatic transmission’, then Covid is inconsequential for such a large number of apparently infected people that they notice no symptoms whatsoever, while others have symptoms so mild they are comparable to the common cold, yet a minority of infected people suffer very severe reactions and unfortunately sometimes lethal outcomes. This very wide variance in how individuals’ bodies react to the virus makes COVID-19 a most unusual illness. What follows are some speculative musings on potential factors influencing individual variance, in other words, asking the question: Have we missed something that could explain why some people fall very ill and even die, yet others don’t even know they’ve got it? Besides individual disparity in reactions to the SARS-CoV-2 virus, there is of course the disparity in how regional variation affects mortality and severe illness. Contrary to most beliefs in an efficient health care system (including preventative care, hygiene, nutrition, immunisation programmes), whereby there should be less illness in those nations that have better and more accessible healthcare provisions, Covid actually seems to be less of a threat to poorer, economically weaker nations which had a lower case fatality rate ...