Viruses are so little that a microscope is required to see them, and their structure is quite basic. When a virus particle is self-contained, it consists of a viral genome, or genetic material, encased in a protein shell known as a capsid. There are three types of viruses: DNA viruses, which include both human and animal viruses; RNA viruses, which include plant viruses; and retroviruses, which include HIV. A virus must make copies of itself for this infection to be successful. This means the virus must find a new cell to infect with its DNA or RNA. A virus can do this by either attaching to a cell's surface or using its special proteins to penetrate the cell membrane and enter the cell. Once inside the cell, the viral DNA or RNA directs the production of more viruses. These new viruses then go on to find other cells to infect! Since viruses are made of molecules of carbon, hydrogen, oxygen, nitrogen, and some other elements, they are actually part of a class of organism called organisms. Organisms are divided into two broad groups: prokaryotes and eukaryotes. Prokaryotes are organisms without true nuclei (their cells contain membranes instead). Eukaryotes have true nuclei and are therefore more complex than prokaryotes.