All viruses have a capsid or head region that contains its genetic material. The capsid is made of proteins and glycoproteins. Capsid contruction varies greatly among viruses, with most being specialized for a particular virus's host organism. Some viruses, mostly of the type infecting animals, have a membranous envelope surrounding their capsid. This allows viruses to penetrate host cells through membrane fusion. The virus's genetical material rests inside the capsid; that material can be either DNA, RNA, or even in some cases a limited number of enzymes. The type of genetic material a virus contains is used in classification, and is discussed in Virus Classification.
In addition to the head region, some viruses, mostly those that infect bacteria, have a tail region. The tail is an often elaborate protein structure. It aids in binding to the surface of the host cell and in the introduction of virus genetic material to the host cell.
Many cells working together forms a tissue. Many tissues working together forms organs. Many organs working together forms organ systems. So it goes from general to most specific. Organ systems are needed in order for life. This proves that cells are the most basic units of life.