Viruses are the smallest organisms, with diameters ranging from 20 nm to 300 nm (1 nm = 10-9 meter). A virus consists of DNA or RNA surrounded by a protein coat called capsid. Some viruses also have an envelope surrounding the capsid. Viruses can be sphere-shaped, or helical.
While many bacteria are not harmful to the human body, most viruses do cause disease because they invade living, normal cells. They then multiply and produce other viruses like themselves. Each virus is very particular about which cell it attacks. Various human viruses specifically attack particular cells in the body's organs, systems, or tissues, such as the liver, respiratory system, or blood cells.
Although types of viruses behave differently, most survive by taking over the machinery that makes a cell work. Briefly, when a single virus particle (virion), comes in contact with a cell it likes, it may attach to special landing sites on the surface of that cell. From there, the virus may inject molecules into the cell, or the cell may swallow up the virion. Once inside the cell, viral molecules such as DNA or RNA direct the cell to make new virus offspring. That's how a virus infects a cell.
Viruses can even infect bacteria. These viruses, called bacteriophages, may help researchers develop alternatives to antibiotic medicines for wiping out bacterial infections.
Some viral infections do not result in disease. For example, by the time most people in the United States become adults, they have been infected by cytomegalovirus (CMV). Most of these people, however, do not develop CMV disease symptoms. Other viral infections can result in deadly diseases, such as HIV which causes acquired immunodeficiency syndrome (AIDS) and coronaviruses which cause severe acute respiratory syndrome (SARS).