It’s a drizzly morning on New York’s Upper East Side, and Rockefeller University microbiologist David Thaler is sipping a double espresso amid the retro-hippie pillows and dangling paper stars of Java Girl, a favorite haunt of the neighborhood’s brainiac Nobel laureates, aging poets and famous entertainers. Thaler somehow manages to embody all three—a long, graying ponytail curling down the middle of his back, wire-frame glasses askew over expansive brown eyes, and a schnozz to rival an Einstein, Ginsberg or Allen. Thaler is one of the leading cheerleaders for a new field of biotechnology aimed at engineering the bacteria inside us to deliver drugs, destroy tumors, actively fight infection, and even vaccinate against their disease-causing kin.

In February, researchers at UCLA announced a clash of the titans, biochemically speaking: They turned one of the great scourges of humankind—HIV—into a hunter of another: cancer. In tests on mice afflicted with metastatic melanoma, a modified strain of HIV invaded cancer cells without infecting the rodents with AIDS.