A virus is basically a gene transporter with the express purpose of infecting another cell in order to replicate. The virus consists of genetic material, either DNA or RNA, protected by a protein shell called a capsid. This capsid is typically self-assembled by proteins created by the viral genetic material. Some viruses however do assist in the construction of the capsid.

The first morphological type is the helical virus. The helical virus has a single type of protein in the shape of an enclosed tube, surrounding the genetic material, that resembles a spiral staircase. This causes the virus to be long and flexible, or short and rigid. The longer a helical virus, the more flexible it must be to prevent forces from snapping it. The length of the virus capsid is dependant on the length of the genetic material and the width of the virus depends on the length and arrangement of the proteins of the capsid.

The second morphological type is the icosahedral capsid. This symmetrical morphology causes the virus to appear spherical under low magnification but it actually contains capsomers, ring shaped structures, that are arranged in a pattern similar to a soccer ball. Capsomers may contain more proteins than the helical virus, a capsomer is constructed from five or six copies of each protein. Icosahedral capsids enclose the viral genetic material less intimately than the helical capsid. The icosahedral form was used by R. Buckminster-Fuller to create the geodesic dome.

The third morphological type is the enveloped virus. This virus, in addition to having a capsid, are able to acquire a modified form of cell membrane from an infected host cell. There are two layers to this membrane, or viral envelope. The inner membrane contains proteins coded by the virus genetic material and proteins coded by the host's genetic material, while the outer layer and any carbohydrates are strictly coded from the host DNA. The viral envelope gives viruses certain advantages over other naked viruses, the most notable of which is the protection the virus gains from the host organism. This envelope causes nearby cells to mistakenly believe the virus is a friendly cell, which results in the virus being taken into the healthy cell. However this envelope has drawbacks for the virus. Many viruses with these envelopes are so dependent upon them that if the envelope is removed, the virus dies.

The fourth and final, morphological type is the complex virus. These viruses do not fall into any of the previous categories and may have extra structures, such as tails or an outer wall. Examples of complex viruses are bacteriophages, viruses that infect bacteria and the poxviruses. Certain bacteriophages have an icosahedral head with a helical tail. The poxviruses have unusual morphology. The viral genetic material is surrounded by a membrane and two bodies with an unknown function and covering the whole thing is an outer envelope studded with a thick layer of proteins.

Viruses vary in size with most being in the range of 100 to 300 nanometers long. As a comparison, a medium sized virus next to a flea is like a human standing next to Mt. Everest. Most viruses are so small that they cannot be seen with a light microscope, so scanning and transmission microscopes are used to get a visual representation of a virus. Viruses contain either RNA or DNA, not both. The exception to this is the human cytomegalovirus, which contains both DNA and mRNA. Animal viruses have one of all combinations, plant viruses tend to have single stranded RNA and bacteriophages have double stranded DNA.