Viruses are any of a number of organic entities consisting of genetic material surrounded by a protective coating. The term virus was first used in the 1890s to describe agents that caused diseases but were smaller than bacteria. By itself a virus is a lifeless form, but within living cells it can replicate many times and harm its host in the process. There are hundreds of known viruses that cause a wide range of diseases in humans, other animals, insects, bacteria, and plants. Virologists tell us that there are probably thousands of viruses that have not yet been discovered.
Viruses are smaller than bacteria and can only replicate within living cells.
Examples of viruses creating problems for humans today are HIV, hepatitis A, B, C and D, herpes Simplex I and II and rotaviruses. Rotaviruses, are among the most important infectious causes of acute diarrhea, frequently causing outbreaks in hospitals, daycare centers, schools, and nursing homes. In the United States, rotaviruses cause about 3.5 million infections every year, resulting in nearly 70,000 hospitalizations and 75 to 125 deaths annually in children. Outbreaks of rotaviral infection are common in infants and young children in hospitals, daycare centers and schools. The elderly also are at a higher risk of acquiring rotaviral infections.