On a microscopic scale, viruses are midgets compared with bacteria. All viruses are submicroscopic, meaning that you can't see them with a standard laboratory microscope. In fact, it was not until the electron microscope was invented in the 1940s that scientists had any notion of what even the largest virus looked like. On the outside, viruses come in several different shapes. On the inside, viruses contain a core of nucleic acids, either DNA or RNA, surrounded by one or two protein shells called capsids.
Many viruses also have an outer envelope made of a mixture of proteins, fats, and carbohydrates.
Yet once a person swallows it or breathes it in, that same virus can invade the person's cells, reproduce, and make the person sick. Viruses do this by replication. They invade living cells and use the cells' internal chemical reactions to duplicate their viral nucleic acids and proteins. Then they assemble new viruses and release them to infect other helpless cells nearby. Many viruses invade the body by crossing the moist mucous membranes that line the breathing passages, eyes, intestines, or reproductive tracts.