With the exception of newly discovered prions, viruses are the smallest agents of infectious disease. Most viruses are exceedingly and essentially round in shape. They consist of little more than a small piece of genetic material surrounded by a thin protein coating. Some viruses are also surrounded by a thin, fatty envelope. Viruses are different from all other infectious microorganisms because they are the only group of microorganisms that cannot replicate outside of a host cell. Because viruses do not eat food - instead they size materials and energy from host cells by hijacking cellular machinery - some scientists argue that they are more like complex molecules than living creatures. Viruses are known to infect nearly every type of organism on Earth. Some viruses, called bacteriophages, even infect bacteria.
Viruses are a cellular, non-cytoplasmic infectious agents. They are smaller than bacteria, and this can pass through bacteriological filter. Viruses are transmissible from disease to healthy organisms. All viruses are obligate parasites and can multiply only within the living host cells. Viruses contain only a single type of nucleic acid either DNA or RNA. Viruses are host specific that they infect only a single species and definite cells of the host organisms. Viruses are effective in very small doses. They are highly resistant to germicides and extremes of physical conditions.