Viruses Health Article

Role in human health

Viruses that infect humans cause damage to the infected cells, resulting in outward symptoms seen as human disease. Human viruses gain entry into the body using various routes. Some viruses are transmitted through skin-to-skin contact, such as herpes simplex 1, the virus that causes cold sores. Others are transmitted through exposure to infected blood, the mode of transmission of the hepatitis B virus. Some of the most easily caught viruses, such as varcella-zoster, the virus that causes chicken pox, are transmitted through water droplets suspended in the air. The virus is transmitted when the droplets are breathed in and come in contact with the respiratory tract of the new host.

Gastrointestinal viruses are transmitted through exposure to waste products containing virus particles that has contaminated water or food, and entry into the host's digestive tract through the mouth. Rotavirus, a cause of a diarrheal illness common in children, is transmitted in this manner. Sexually transmitted viruses move from host to host through sexual contact and enter the body by the genitourinary route. HIV and human papilloma virus (HPV) are examples of viruses that are sexually transmitted.

After gaining entry into the host, the response at a cellular level to the viral infection varies with the type of virus and the virulence of the strain. Thus, the response can vary from no apparent change, to detectable changes in the cell, known as cytopathic effects (CPE), to loss of growth control or malignancy. Virulence refers to the ability of a virus to cause disease in a host. Some viruses are highly virulent, causing disease with almost every infection. Measles, rabies, and influenza are virulent viruses. Other less virulent viruses, such as Epstein-Barr virus, which causes mononucleosis, only rarely results in disease symptoms.

Viral infections follow patterns that are specific to the virus. Some infections are localized, that is, restricted to a particular cell type or organ, while others are disseminated throughout the body. Disseminated infections are often propagated through the nervous system or the bloodstream. Infections can be acute, where the patient's immune system self-limits the disease and recovers, or chronic, where the infection continues for a long period of time.

Some viruses have the ability to cause an initial disease state upon infection and then establish a latent or dormant infective state. For example, herpes viruses cause blisters on the skin as a result of their lytic replication, but then establish a latent infection in nerve cells. Upon a stimulus such as exposure to the sun or stress the virus switches back to a lytic cycle, again producing blisters at the site of infection. In this way, the infection can persist for months or even years.

Several viruses, such as human papilloma viruses and Epstein-Barr virus, have been strongly associated with human cancers. The exact role of viruses in malignancy is not yet understood, as environmental and host genetic factors also seem to contribute to the development of tumors. However, it is highly probable that viruses are key triggers for a number of human cancers.

Another effect of viruses on human health is infection by zoonotic viruses, that is, viruses that can be transmitted from an animal host of another species to humans. Some of these viruses are transmitted through a blood-sucking insect intermediary, such as a mosquito, while others are transmitted directly from the infected animal to humans. Many of these viruses raise important public health concerns. An example of a mosquito-transmitted virus is flavivirus that causes West Nile encephalitis in humans. A strain of hantavirus was discovered in 1993 that infects rodents and transmits directly to humans, causing a respiratory illness.

Although most infect plants and animals, a few unconventional viruses cause human disease. The only know human viroid is the delta virus (hepatitis D) that requires co-infection with hepatitis B to be infective. The combined infection of hepatitis B and D causes more severe symptoms than B alone. An example of a human prion-mediated disease is Cruetzfeldt-Jakob disease (CJD), which causes neurological symptoms and is fatal. Of significant concern is a possible variant of CJD reported in Great Britain that affects younger individuals. Although cause and effect has not been conclusively shown, there is suspicion that this disease results from eating beef contaminated with the prion that causes bovine spongiform encephalopathy, or mad cow disease.