Kinds of viral infections

Diseases caused by viruses range from minor infections that may go unnoticed to serious diseases, such as hepatitis and AIDS. Some viruses cause major disturbances in cells growth and reproduction, resulting in tumors or cancer.

Viruses tend to attack particular species of animal or plant and a specific type of cell within the organism. A cold virus, for example, attacks cells of the respiratory system and a poliovirus invades nerve cells. Many viruses attack more than one species. Rabies, for example, can be transmitted from dogs, raccoons, foxes, and bats to other mammals, including humans. Cowpox can be transmitted to humans handling diseased herds. The virus responsible for a pneumonia like disease called psittacosis can travel to humans from parrots and other birds infected with the disease.

Infections produced by viruses that undergo the lysogenic cycle may remain latent for a long period, and then become active again. One example is the recurring infection caused by the herpes simplex virus. One type of herpes causes cold sores and another type causes genital sores. These sores disappear during the inactive, lysogenic cycle and appear during the virulent lytic cycle.

The ability to cause disease is called virulence. Several factors determine the virulence of a virus. One factor is the presence and activity of receptor sites on the surface of the cell which enable the virus to become attached. Another factor is the speed with which the virus multiplies once it penetrates the host cell. A third factor is the response of the organism to the invading viruses. For example, the cell die immediately, it may divide abnormally or it may produce defenses against the viruses.

Date: 2014-12-22; view: 1035