IL15 and viral infectious disease: Based on our results in Figures 1–2 showing increased levels of circulating activated/proliferating CD4+ and CD8+ T-cells in severe patients and on the elevated levels of cytokines involved in T-cell proliferation (IL-7, IL-15, TNF-α) at 3 months, we asked whether factors present in the plasma of severe patients might render T-cells more susceptible to ‘bystander T-cell’ activation, which occurs during viral infection and is believed to be driven by IL-15.