A virus is a tiny submicroscopic viral agent which replicates within the living cells of a living organism. Most viruses infect all living life forms, including plants and animals to human beings, including archaenians and yeast. It can be single-celled or multicellular; the two types have different ways of replication. The majority of viruses are composed of genetic material called protein, which encodes their protein. The protein is replicated by an enzyme called a ribosome, with instructions on how the virus should multiply inside of living cells.


In order for a virus to replicate, it must invade a living host cell through the puncture of a virus-bearing envelope. At this juncture, the virus gets into the host cell’s DNA by either engulfing the host cell’s membrane via the viral envelopes, or intruding within as a directed RNA sequence. The nucleic acid of the virus then copies itself into a messenger RNA or mRNA, which then instructs the production center of the cell to duplicate the virus genome. A virus then inserts into the host cell’s chromosome, creating a new set of genetic instructions for the cell’s various functions. Once the virus has entered the host cell, the genetic information contained in the viral genome is continually replicated throughout the entire cell.

Viruses are responsible for the majority of pathogenicity in living systems. The majority of viruses cause a variety of disease states, ranging from infections which only cause minor symptoms, such as colds and the common cold to serious infections which require extensive medical intervention, including immunosuppression therapies to cure. Pathogens can affect any living thing, although they tend to concentrate most commonly on living things which are warm-blooded. Human beings are the principal reservoir of pathogenic viruses, and all forms of bacteria, fungi and parasites can produce infectious agents. Virtually all viruses are killed by the natural immunity systems of the host organism, although occasionally they survive to cause a new strain.

There are many different classes of viruses and their hosts. Some viruses are necessary for the development of various organisms and some are detrimental to an organism’s development, but viruses that invade an animal’s host cells are particularly dangerous. The major class of viruses that invade host cells is the genus of retroviruses, which includes a number of varieties that have been identified to cause major outbreaks of disease in humans. These include the common cold, influenza, SARS and herpes. Other important viruses include Polirritmo virus, Lassa virus, Restless leg syndrome virus, and Epstein-Barr virus.

A number of bacteria are also capable of producing their own infectious agents and are capable of invading living cells. Some examples of these are the Streptococcus species, which causes strept throat; and Staphylococcus aureus, which is responsible for infection of the breast and rectum. HIV and AIDS are the other notable bacterium viruses. The HIV virus is responsible for the acquired immune deficiency syndrome or AIDS, while the AIDS virus is the causative agent of acquired cellular immune deficiency syndrome or ACIDS AIDS. All these viruses affect specific sets of living cells.

Similarly, there are many viral diseases that affect genetic material in living organisms. These include genetic viruses that affect cellular DNA; retroviruses that affect the genetic material found within RNAs; and viruses that affect RNA viruses (miRNAs). In addition, there are microbial viruses that infect single cell organisms, such as bacteria and viruses of the Streptococcus species. The major varieties of microbial viruses are listed in the Staphylococcus species and include: Streptococcus viridans, Staphylococcus aureus, Staphylococcus evaluata, and Staphylococcus salivarius.