A bacteriophage, or phage, is a virus that infects a bacterial cell, taking over the host cell's genetic material, reproducing itself, and eventually destroying the bacterium. The word phage comes from the Greek word phagein, meaning "to eat." Bacteriophages have two main components, protein coat and a nucleic acid core of DNA or RNA. Most DNA phages have double-stranded DNA, whereas phage RNA may be double or single-stranded. The electron microscope shows that phages vary in size and shape. Filamentous or threadlike phages, discovered in 1963, are among the smallest viruses known. Scientists have extensively studied the phages that infect Escherichia coli (E.coli), bacteria that are abundant in the human intestine. Some of these phages, such as the T4 phage, consist of a capsid or head, often polyhedral in shape, that contains DNA, and an elongated tail consisting of a hollow core...