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Viruses are moochers. They contain only a limited genetic blueprint and they don't have the necessary building tools. They have to invade other cells and hijack their cellular machinery to reproduce. Viruses invade by attaching to a cell and injecting their genes or by being swallowed up by the cell.
Pictured above are T4 bacteriophages (back-tear-e-oh-faj-es). They are a kind of virus that infects bacteria. Here they are landing on the surface of an E. coli bacterium.
The bacteriophage cuts a hole in the E. coli's cell wall. It then injects its genetic material into the bacterium. By taking over the E. coli's genetic machinery, the viral genes tell the bacterium to begin making new virus parts. These parts come together to make whole new viruses inside the bacterium.
Eventually so many new viruses are made that the E. coli bursts open and dies, releasing all those new viruses to infect more cells!