Virus

An extremely small infective agent, visible only with an electron microscope. Viruses can cause disease in humans, animals and plants. Viruses consist of a protein coat and either a DNA or RNA strand of genetic material. Viruses can reproduce only inside a host cell by using the cell's machinery to reproduce. Thus, viruses are considered either the simplest type of organism or a complex, non-living, molecule. Viruses are not killed by antibiotics.