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“Sir, I am entirely lysed!” shouted a technician in André Lwoff's laboratory at the Institut Pasteur in 1949, describing cells undergoing lysogenic induction. Phages (short for bacteriophages) are DNA or RNA viruses that infect only specific bacteria. Phages have colorful names, such as λ, T4, SopEΦ, Mu, Qβ, P1, or ΦX174; they come in many shapes and sizes (e.g., tailed or nontailed, icosahedral, filamentous, and enveloped or nonenveloped); they have many different lifestyles (e.g., lytic or temperate); and they can carry genes that increase the pathogenesis of bacteria. They are among the most intensely studied viruses and are perhaps best . . .