Infections caused by bacteria are called bacterial infections. Viral infections are caused by viruses. Most bacteria will be killed or kept from growing by antibiotics, but viruses will not be treated by them. Some viruses can be removed from the body with the help of antiviral medicines. A bacteria is a single-celled microorganism. There are many types of environments in which they can live. It is also possible for bacteria to live on and in the human body. In the majority of cases, bacteria do no harm and, in some cases, they can be helpful. Bacteria in the intestines, for instance, assist in digestion. It is possible, however, for bacteria to cause illness. A urinary tract infection can be caused, for example, by bacteria that travel from the anus to the urinary tract. Viral particles consist of genetic information, either RNA or DNA, wrapped in protein. Viral infections need living hosts, such as people, plants, or animals. In order to spread, a virus must enter a host's body and then reach its cells. After that, it takes over the host cell's machinery and uses it to produce more viruses.