This document discusses bacteriophage T4, a virus that infects E. coli bacteria. It has a complex protein coat and large double-stranded DNA genome. T4 uses the host cell's machinery to replicate and kills the host cell. T4 plays a role in cholera and diphtheria by carrying toxin genes that allow the bacteria to cause disease. Bacteriophage may be useful for treating antibiotic-resistant bacteria or infections where antibiotics cannot reach. T4 is also used in recombinant DNA technology.