Virus (Continued)..

Examples of Viruses and the Diseases they cause:

T4 Bacteriophage This transmission electron micrograph shows a T4 bacteriophage, a virus that infects only bacteria (and in this case only Escherichia coli). Phages lack any reproductive machinery and rely on the apparatus of bacteria in order to replicate. They do so by attaching to the cell wall of the bacterium with the spidery tail fibres visible here. The tail is a sheath that contracts to inject the contents of the head, the genetic material (DNA), into its host. Within 25 minutes of infection, the bacterial apparatus successfully commandeered, viral progeny fill the cell. The overcrowded bacterium bursts, releasing approximately 100 new copies of the bacteriophage.

Adenovirus.This icosahedral adenovirus particle measures 75 nm in diameter. It is protected by the capsid (shown here in red)—a "shell" or "coat" consisting of two types of protein. The protein subunits (capsomeres) are visible here as small, circular shapes. The spikes protruding from each vertice on the capsid surface interact with cell receptors and determine the virus's infective properties. At the core of the virus particle its DNA is contained, associated with a third type of protein different from those in the capsid.
This is a false colour photograph of an adenovirus particle seen under an electron microscope.

The Human Immunodeficiency Virus (HIV), which causes acquired immune deficiency syndrome (AIDS), principally attacks T-4 lymphocytes, a vital part of the human immune system. As a result, the body’s ability to resist opportunistic viral, bacterial, fungal, protozoal, and other infection is greatly weakened. Pneumocystis carinii pneumonia is the leading cause of death among people with HIV infection, but the incidence of certain types of cancers such as B-cell lymphomas and Kaposi’s sarcoma is also increased. Neurological complications and dramatic weight loss, or “wasting”, are characteristic of end-stage HIV disease (AIDS). HIV is transmitted sexually, through contact with contaminated blood, tissue, or needles, and from mother to child during birth or breast-feeding. Full-blown symptoms of AIDS may not develop for more than ten years after infection.

Rabies Virus.The rabies virus is usually transmitted to humans by a bite from an infected dog, but the bite of any animal (wild or domestic) is suspect in an area where rabies is present. Symptoms of the disease appear after an incubation period of ten days to one year and include fever, breathing difficulties, muscle spasms, and, in later stages, an irrational fear of water. Death almost invariably occurs within three days to three weeks of the onset of symptoms. For this reason, the emphasis of treatment is on prevention. Dogs may not be brought into the United Kingdom until a lengthy period of quarantine has elapsed, while in the United States, domestic dogs are vaccinated yearly and stray dogs are killed.

Hepatitis B Virus.(HBV) causes inflammation of the liver. The virus is recognizable under magnification by the round, infectious “Dane particles” accompanied by tube-shaped, empty viral envelopes. Manifestations of this condition include jaundice and a flu-like illness, while chronic infection can lead to serious pathologies such as cirrhosis and cancer of the liver.