Virus, infectious agent found in virtually all life forms, including humans, animals, plants, fungi, and bacteria. Viruses consist of genetic material — either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) — surrounded by a protective coating of protein, called a capsid, with or without an outer lipid envelope. Viruses are not considered free-living, since they cannot reproduce outside of a living cell; they have evolved to transmit their genetic information from one cell to another for the purpose of replication.

Viruses often damage or kill the cells that they infect, causing disease in infected organisms. A few viruses stimulate cells to grow uncontrollably and produce cancers. Although many infectious diseases, such as the common cold, are caused by viruses, there are no cures for these illnesses. The difficulty in developing antiviral therapies stems from the large number of variant viruses that can cause the same disease, as well as the inability of drugs to disable a virus without disabling healthy cells. However, the development of antiviral agents is a major focus of current research, and the study of viruses has led to many discoveries important to human health.

The first contact between a virus particle and its host cell occurs when an outer viral structure docks with a specific molecule on the cell surface. After binding to an appropriate cell, a virus must cross the cell membrane.
Once inside the cell, the virus replicates itself through a series of events. Viral genes direct the production of proteins by the host cellular machinery. The first viral proteins synthesized by some viruses are the enzymes required to copy the viral genome. Using a combination of viral and cellular components, the viral genome can be replicated thousands of times. To complete the virus replication cycle, viruses must exit the cell.
The virus replication cycle can be as short as a couple of hours for certain small viruses or as long as several days for some large viruses.
Cells may survive virus infection, and the virus can persist for the life of its host. Virtually all people harbor harmless viruses.

Most viral infections cause no symptoms and do not result in disease. In contrast, most people who are infected with measles, rabies, or influenza viruses develop the disease. A wide variety of viral and host factors determine the outcome of virus infections. A small genetic variation can produce a virus with increased capacity to cause disease. Such a virus is said to have increased virulence.
Viruses can enter the body by several routes :
• Through the skin by direct contact with virus-containing skin lesions on infected individuals : Herpes Simplex Virus and pox
• Infected blood products : Ebola, hepatitis B, HIV
• Through the respiratory tract ( via airborne dropledts of mucus or saliva from infeted individuals who cough or sneeze) : influenza, rhinovirus and adenovirus ( common cold), and varicella-zoster virus ( chicken pox)
• Through the gastrointestinal tract : rotavirus, coronavirus, poliovirus, hepatitis A
• Through the genitourinary route : HPV, HIV, Herpes simplex
• Through the eye : echovirus, Coxsackie virus, herpesvirus

Virus-induced illnesses can be either acute, in which the patient recovers promptly, or chronic, in which the virus remains with the host or the damage caused by the virus is irreparable. For most acute viruses, the time between infection and the onset of disease can vary from three days to three weeks. In contrast, onset of AIDS following infection with HIV takes an average of 7 to 11 years. 
Several human viruses are likely to be agents of cancer, which can take decades to develop. The precise role of these viruses in human cancers is not well understood, and genetic and environmental factors are likely to contribute to these diseases. But because a number of viruses have been shown to cause tumors in animal models, it is probable that many viruses have a key role in human cancers.

Defense :
The body’s immune system has many natural defenses against virus infections. Infected cells produce interferons and other cytokines (soluble components that are largely responsible for regulating the immune response), which can signal adjacent uninfected cells to mount their defenses, enabling uninfected cells to impair virus replication. Some cytokines can cause a fever in response to viral infection; elevated body temperature retards the growth of some types of viruses. B lymphocytes produce specific antibodies that can bind and inactivate viruses. Cytotoxic T cells recognize virus-infected cells and target them for destruction. However, many viruses have evolved ways to circumvent some of these host defense mechanisms.