A virus—unlike a bacteria or chemical pathogen—could spread through a population with astounding speed and penetration. Parasitic in their behavior, viruses entered an organism and attached to a host cell in a process called adsorption. They then injected their own DNA or RNA into that cell, recruiting the invaded cell, and forcing it to replicate multiple versions of the virus. Once a sufficient number of copies existed, the new virus particles would kill the cell and burst through the cell wall, speeding off to find new host cells to attack, and the process would be repeated. An infected
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