Viruses

A virus is an extremely small infectious agent—much smaller than a fungus or bacterium—that must invade a living cell to reproduce (replicate). The virus attaches to a cell, enters it, and releases its genetic material inside the cell. The viral genetic material takes control of the cell and forces it to replicate the virus. The infected cell usually dies because the virus keeps it from performing its normal functions. Before it dies, however, the cell releases new viruses, which go on to infect other cells.
Viruses occur throughout the biosphere. Cells of all living organisms are infected by a variety of viruses that considerably outnumber the host cells. Viruses usually infect one particular type of cell. For example, cold viruses infect only cells of the upper respiratory tract. Additionally, most viruses infect only a few species of plants or animals; some infect only humans.

Enveloped viruses

Although viruses have adapted to different host systems during evolution and many different viral strategies have developed, certain similarities can be found. For instance, many of the highly pathogenic viruses including influenza virus, HIV and others of worldwide epidemiological importance are said enveloped, meaning that they are surrounded by a lipid membrane. This viral membrane, which originates from the host cell during the exit process, is required to protect the viral genome and provide important functions for attachment, morphogenesis and transmission.

Mode of transmission

Viruses are transmitted in a variety of ways. Some are swallowed, some are inhaled, some are transmitted by the bites of insects and other parasites (for example, mosquitoes and ticks) and some are transmitted by sexual intercourse.
Probably the most common viral infections in humans are those of the nose, throat, and airways. These infections include sore throat, sinusitis, the common cold, and influenza. These diseases are often termed upper respiratory infections (URIs). In small children, viruses also commonly cause croup and inflammation of the windpipe (laryngitis) or other airways deeper inside the lungs (bronchiolitis, bronchitis).