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Most people who contract the dengue virus, a mosquito-borne RNA virus, experience mild symptoms or none at all. In some cases, it can cause a severe illness known as hemorrhagic fever, with bleeding, abnormal blood clotting, and leaky blood vessels that can sometimes lead to a precipitous drop in blood pressure and circulatory collapse. Curiously, in the 1960s, US army scientists in Thailand noticed this life-threatening condition occurred most frequently in two populations: first-time infected babies born to mothers who were immune to dengue, and children who had once experienced a mild or asymptomatic infection, and later contracted the virus a second time. A scary scenario began to crystalize: a second infection was sometimes worse than the first.
A series of studies in cells, animals, and people eventually gave rise to a possible explanation: antibodies created during a first-time infection could, under very specific circumstances, end ...
