In the first of a new column, a medical doctor explains how the immune system changes after middle age and causes ageing’s big diseases. Words: Dr Roderick Mulgan Covid-19 has hit some people worse than others, mainly those with pre-existing health conditions. But even before the pandemic our lifespan, as well as our health span (years of healthy living), came down to dodging the significant diseases of later life for as long as possible: heart attacks, strokes, cancer and dementia, to cite the most dramatic. It is relatively recent — the past 100 years or so — that ageing stopped