Once inside a host cell, viruses take over its machinery to reproduce. Viruses override the host cell’s normal functioning with their own set of instructions that shut down production of host proteins and direct the cell to produce viral proteins to make new virus particles.
Some viruses insert their genetic material into the host cell’s DNA, where they begin directing the copying of their genes or simply lie dormant for years or a lifetime. Either way, the host cell does all the actual work: the viruses simply provide the instructions.
Viruses may be able to infect and reproduce in more than one kind of animal, but the same virus can cause different reactions in different hosts.
For example, flu viruses infect birds, pigs, and humans. While some types of flu viruses don’t harm birds, they can overwhelm and kill humans.
Plant viruses do not infect animals or vice versa. Viruses that infect bacteria do nothing to animal or plant cells.