Viruses are prolific. They have been found in almost all parts of the Earth, including the Arctic. They infect nearly every form of life from the simplest unicellular organism (bacteria) to complex mammals, such as humans. The structures of all human viruses have several common attributes. First is their incredibly small scale. There are 10 x10+9 nanometers in a meter. Viruses are so minuscule that they can only be visualized using electron microscopes. Another common structural property of viruses is that they are constructed from protein subunits. Subunits are simply identical proteins.
The virus is therefore constructed of many copies of one or several types of subunits. This reduces the amount of genetic material needed and consequently, the amount of possible mutations. Viruses or viral diseases: filovirus (Ebola or Marburg), hantavirus, hepatitis A, hepatitis B, hepatitis C, influenza, smallpox, yellow fever, polio, common cold, Epstein-Barr, measles, rabies, rubella. The most significant property of viruses is that they are not self-reproducing. They need the presence of another organism or host to reproduce themselves, or replicate.
This host possesses the machinery that the virus needs to replicate. Specifically, the host possesses ribosomes, which the virus itself cannot synthesize. Essentially, viruses are cellular parasites. Their life and the life of their offspring depend on the host. Another distinguishing property of viruses is that they contain only one type of nucleic acid. This is in contrast to the host cell, which always has two types (DNA and RNA). The last property of viruses is the fact that they are reproduced solely from their nucleic acid, whereas the host cell is reproduced from the integrated sum of its components.
The virus never arises directly from a pre-existing virus, whereas the host cell always arises directly from a pre-existing cell. Experiments have clearly shown that the components of a virus are synthesized independently and then assembled into mature virus particles. In contrast, growth of the host cell consists of an increase in the amount of all its parts, during which the individuality of the cell is continuously maintained. The outcome of viral infection is dependent upon the viral state.
If the virus is in a lytic state, the host cell is destroyed, releasing progeny virus into the surrounding medium. In lysogeny, the virus integrates its genome into the host genome. At a later date, the viral genome may be activated and can enter a lytic state. But most of the time, the viral genome just stays silent in the host cell. In latency, a form of lysogeny, the viral genome stays in the host cell, but is not necessarily incorporated. The genome is present but does not produce many products. Again, the viral genome can be reactivated, generating disease in the host.