A virus is a small infectious organism transmitted through either sexual contact, inhalation, swallowing or insect bites, according to the Merck Manual. The most common way in which viruses spread is by throat, airway and nasal passage infection.
Viruses can infect both plants and animals, with some infecting only humans, reports the Merck Manual. Some viruses primarily infect mainly infants and children. Viruses usually are attracted to one particular cell type and sometimes exist only in certain regions of the world. Viruses can infect those located in other areas for a number of reasons, such as global warming and travelers who transport and transmit the infections to those in other areas of the world.
Viruses replicate using either DNA or RNA once it attacks a host cell, says the Merck Manual. Inside the host, the virus overthrows cell operation, forcing the host cell to replicate the virus’ genetic material. The virus either changes the function of the host cell or kills it. When the host cell dies, more viruses release into the body, attacking the next host cell and beginning its destruction of subsequent host cells.
Some host cells with altered cell function become cancerous, losing control over cellular division, explains the Merck Manual. Viruses, such as human immunodeficiency virus, insert genetic material in the cell, causing the infection to remain in the cell dormant for some time until disturbed. The disturbance triggers the infection cycle of replication and invasion throughout the body.