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- EFFECTS OF VIRUSES -
The range of structural and biochemical effects that viruses have on the host cell is extensive. These are called cytopathic effects. Most virus infections eventually result in the death of the host cell. The causes of death include cell lysis, alterations to the cell's surface membrane and apoptosis. Often cell death is caused by cessation of its normal activities because of suppression by virus-specific proteins, not all of which are components of the virus particle. Some viruses cause no apparent changes to the infected cell. Cells in which the virus is latent and inactive show few signs of infection and often function normally. This causes persistent infections and the virus is often dormant for many months or years. This is often the case with herpes viruses. Some viruses, such as Epstein-Barr virus, can cause cells to proliferate without causing malignancy, while others, such as papillomaviruses, are established causes of cancer.
Unlike a bacterium or a cell of an animal, a virus lacks the ability to replicate on its own. A virus does contain some genetic information critical for making copies of itself, but it can't get the job done without the help of a cell's duplicating equipment, borrowing enzymes and other molecules to concoct more virus. Samples of common human diseases caused by viruses include the common cold, influenza, chickenpox and cold sores. Many serious diseases such as Ebola, AIDS, avian influenza and SARS are caused by viruses. The relative ability of viruses to cause disease is described in terms of virulence. Here are a few types of common viruses and some major viruses that have infected Singapore since the past years. Influenza: Commonly known as flu, is an infectious disease caused by RNA viruses of the fam Influenza A virus; Wild aquatic birds are the natural hosts for a large variety of influenza A. Occasionally; viruses are transmitted to other species and may then cause devastating outbreaks in domestic poultry or give rise to human influenza pandemics. The type A viruses are the most virulent human pathogens among the three influenza types and cause the most severe disease. The influenza A virus can be subdivided into different serotypes based on the antibody response to these viruses. The serotypes that have been confirmed in humans, ordered by the number of known human pandemic deaths, are:
 Hand, foot and mouth disease (HFMD): is a human syndrome caused by intestinal viruses of the Picornaviridae family. HFMD usually affects infants and children, and is quite common. It is moderately contagious and is spread through direct contact with the mucus, saliva, or feces of an infected person. It typically occurs in small epidemics in nursery schools or kindergartens, usually during the summer and autumn months. The usual incubation period is 3–7 days. Symptoms of HFMD include:
Early symptoms are likely to be fever often followed by a sore throat. Loss of appetite and general malaise may also occur. Between one and two days after the onset of fever, painful sores (lesions) may appear in the mouth and/or throat. A rash may become evident on the hands, feet. Not all symptoms will be present in all cases. Human immunodeficiency virus (HIV): is a lentivirus that causes acquired immunodeficiency syndrome (AIDS), a condition in humans in which the immune system begins to fail, leading to life-threatening opportunistic infections. Infection with HIV occurs by the transfer of blood, semen, vaginal fluid, pre-ejaculate, or breast milk. Within these bodily fluids, HIV is present as both free virus particles and virus within infected immune cells. The four major routes of transmission are unsafe sex, contaminated needles, breast milk, and transmission from an infected mother to her baby at birth (Vertical transmission). Screening of blood products for HIV has largely eliminated transmission through blood transfusions or infected blood products in the developed world. HIV infects primarily vital cells in the human immune system. First, direct viral killing of infected cells; second, increased rates of apoptosis in infected cells; and third, killing of infected CD4+ T cells by CD8 cytotoxic lymphocytes that recognize infected cells. When CD4+ T cell numbers d  Scanning electron micrograph of HIV-1 (in green) budding from cultured lymphocyte. Multiple round bumps on cell surface represent sites of assembly and budding of virions.
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