A virus that infects bacteria, sometimes used as a vector. The lambda bacteriophage is frequently used as a vector in recombinant gene experiments. Examples include dsDNA phages with contractile tails, such as T4; dsDNA phages with long “flexible tails, such as λ, which are most commonly used in DNA cloning; dsDNA phages with stubby tails, such as p22; ssDNA phages, such as phi X 174; and ssRNA phages, such as MS2.