Some viruses are pathogenic, meaning that they cause disease in the organisms they infect.
A viral disease is any viral infection that harms the host.
Many well-known diseases are caused by viruses, such as influenza, AIDS, chicken pox and glandular fever.
Viruses cause disease by damaging and killing a large number of cells in the host organism.
Disease symptoms can also be caused by an auto-immune response. Inflammation of infected and neighbouring cells occurs due to the organism's immune response.
Some viruses can remain dormant within cells for years, and only cause disease under certain conditions.
The varicella zoster virus, which causes chicken pox, remains dormant in the spinal cord even after the disease is cured. The virus can reactivate and cause the disease shingles.
The human immunodeficiency virus (HIV) is the virus that causes acquired immunodeficiency syndrome (AIDS), leading to the progressive destruction of the immune system.
An estimated 34 million people have contracted HIV since 1980 and around 30 million people have died from AIDS related illnesses.
HIV targets T cells, which are specialised white blood cells with a regulatory role in the immune system.
HIV infection ultimately causes a huge drop in the number of T-cells circulating in the blood stream. When the number of T cells drops below a certain level, the immune system becomes compromised and the person develops AIDS.
HIV is a virus that has emerged within the last century. It is thought to have evolved from a virus that infected monkeys to a virus that can infect humans. Emerging viruses are a significant challenge to science and medicine.
HIV has a high mutation rate.
This poses a problem to the body for several reasons:
It makes it very difficult for our body to recognise the HIV antigens (type of glycoprotein) on the cell surface; they change shape regularly making the body unable to build immunity.
The high mutation rate makes it easy for the virus to evolve immunity to drugs.