Immunology ostensibly began with ancient Greek physicians, who marvelled at their observation that patients who had survived a bout of plague were resistant to the disease when it came around again. From this germ was born the idea of vaccination — a small controlled infection, like a sort of immunological rehearsal, designed to confer future resistance to full-blown infection while causing only mere discomfort, rather than disease. What those Greeks also initiated was the tendency for immunologists to tackle their subject in a backward manner. They had become captivated by immunological memory, a late-acting feature of the immune response that only becomes useful if and when the first battle between person and pathogen — the primary infection — has been won by other means.

Memory is the end point for adaptive immunity, the part of defence that is mediated by white blood cells called B cells and T cells. Although fascinating to immunologists for their strength and specificity, these soldiers are not speedy. Typically found in a state of suspended animation, B and T cells only become fit to fight about a week after combat begins. During this first week of a primary infection, defence is in the hands of innate immunity. One of several paths can be followed: the infection can gain the upper hand, leading to death or chronic, debilitating disease; alternatively, an acute infection can cause a temporary state of disease, which ends when innate immunity calls up reinforcements in the guise of adaptive immunity. Patients who experience either of these outcomes have been carefully studied by physicians ever since the time of the ancient Greeks.

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