What Is a Virus?
If you have read How Cells Work, you know how both bacteria cells and the cells in your body work. A cell is a stand-alone living entity able to eat, grow and reproduce. Viruses are nothing like that. If you could look at a virus, you would see that a virus is a tiny particle. Virus particles are about one-millionth of an inch (17 to 1,000 nanometers) long [source: Milo and Phillips].
Most viruses are many times smaller than bacteria, though giant viruses — first discovered in 2003 — are about the same size [source: Pennisi]. Bacteria, in turn, are much smaller than most human cells. Viruses are so small that most cannot be seen with a light microscope, but must be observed with an electron microscope.
A virus particle, or virion, consists of the following:
- Nucleic acid - set of genetic instructions, either DNA or RNA, either single-stranded or double-stranded (see How Cells Work for details on DNA and RNA)
- Coat of protein - surrounds the DNA or RNA to protect it
- Lipid membrane - surrounds the protein coat (found only in some viruses, including influenza; these types of viruses are called enveloped viruses as opposed to naked viruses)
Viruses vary widely in their shape and complexity. Some look like round popcorn balls, while others have a complicated shape that looks like a spider or the Apollo lunar lander.
Unlike human cells or bacteria, viruses don't contain the chemical machinery (enzymes) needed to carry out the chemical reactions for life. Instead, viruses carry only one or two enzymes that decode their genetic instructions. So, a virus must have a host cell (bacteria, plant or animal) in which to live and make more viruses. Outside of a host cell, viruses cannot function. For this reason, viruses tread the fine line that separates living things from nonliving things. Most scientists agree that viruses are alive because of what happens when they infect a host cell.