All viruses consist of an outer protein coat and genetic material (DNA or RNA). The genetic material is the inner, curly, light-blue structure in most of the viruses shown in the image. Because viruses lack most of the structures of a typical cell, especially a nucleus, their cellularity is acellular. Because viruses cannot survive independent of a host, they are not considered to be "alive." Therefore, viruses aren't considered organisms; rather, they are considered agents that cause disease. As a result, viruses are not classified into a kingdom. They depend on a host organism to supply it with the necessary materials to sustain life. Vaccines, which are substances used to improve immunity, can be used to prevent a life-threatening viral infection. Doctors can also prescribe medicines called antivirals to treat specific viral infections.