David Vetter was born without a functioning immune system and spent his life in a bubble that protected him from germs. He died at age 12 in 1984. Scientists are using gene therapy to treat the disorder so that children can live normally. Science Source hide caption

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A 3-D model of HIV peeled back to show its layers. HIV's genetic material sits inside a spherical shell (gray matrix) studded with spikes (dark gray and orange). The sphere pops open when a T cell tugs on a spike. Courtesy of Ivan Konstantinov/© Visual Science 2011 hide caption

itoggle caption Courtesy of Ivan Konstantinov/© Visual Science 2011