Many biologist do not consider a virus to be a living organism. Alone, a virus is not orgasmic, it is not capable of engaging in life processes. It is only when the virus invades a living cell that it shows signs of life.
Once viruses invade living cells they can cause disease. In fact, the word "virus" is the Latin word for "poison." Some diseases that are caused by viruses are: Acquired Immune Deficiency Syndrome (AIDS), polio, small pox, chicken pox, warts, mumps, influenza (flu), rabies, and the common cold. There are more than 300 viruses that are known to cause disease. These are called pathogenic viruses. The word "pathogenic" means capable of causing disease. However, many if not most viruses do not cause disease.
A virus does not have a cell, but is described as being a particle. A virus has no nucleus, no cytoplasm and is not surrounded by a membrane. Viruses are so small that they pass through the filters that are used to separate bacteria from liquids such as milk or beer. Only very large viruses can be seen with the light microscope. Most require an electron microscope to be seen. Viruses are measured by the millimicron. A millimicron is one millionth of a millimeter. Compared to viruses bacteria are giants. The existence of viruses were not proven until this century when the electron microscope was invented.
The simplest viruses are a core of nucleic acid wrapped in a protein coat. Sometimes the nucleic acid is RNA and sometimes it is DNA. More complex viruses can have carbohydrates, lipids, metals and other substances.
The living cell which is invaded by a virus is called the host cell. A virus cannot reproduce on its own. In a living cell, the virus injects its DNA or RNA into the host cell's nucleus. This causes the host cell to become a viral factory.
Viruses generally invade only specific kinds of cells. Some viruses will only invade the kidney. Others only the lungs. Others only skin cells. Others only the heart, and others only the nervous system. The polio virus, for example would attack only nerve cells in the spinal cord and brain. The mumps virus infects only one pair of salivary gland and never any of the others.
Because viruses infect cells by injecting their RNA or DNA into host cells, it has long been suspected that viruses may be a primary cause of cancer. This is even more strongly believed today. It now known that viruses do cause specific cancers, such as certain kinds of leukemia, for example.
There are no drugs which will effectively kill a virus. However, the human body is capable of making white blood cells which will destroy most viruses. The problem in viral disease is that these white blood cells require time to be made. During this time the virus can consume and kill.
The drugs which kill bacteria kill them by destroying their cell membrane. Since viruses do not have a cell membrane, the best that can be done is to create drugs such as AZT, for example, that will slow the reproduction of the virus. However, viral disease can be prevented with vaccine.
A vaccine is prepared by weakening or killing the virus and then injecting this virus into the person. The human body does not know that they are weakened or dead, so they latch on to them to get the information needed to make a white blood cell which will kill the virus. Once the white blood cell has been made, it is available to kill the virus should it invade the body. It was in such a way that vaccines for polio, rabies, measles, small pox, chicken pox, mumps, and influenza were developed. However, the virus which causes the common cold mutates so quickly and so often that to date there is no effective vaccine for the common cold.
Presently there is no vaccine or cure for the AIDS virus. The AIDS virus is contracted in many ways, but it is usually contracted through contact with the bodily fluids of an infected person. Some specific ways that it is contracted are through sexual activity and blood transfusions from blood donated by a person who carries the virus. However, blood screening which occurs today makes it very unlikely that you will get AIDS from a blood transfusion. So many hemophiliacs have AIDS because the blood used to extract the clotting factor which allowed them to treat their bleeding was not properly screened. Today this occurrence is less likely as well.
When a virus enters the human body and gets into the blood stream, the immune system goes into action. The virus may be eaten immediately by the various white blood cells which patrol your body's blood stream.
However, if you lack a white blood cell which will kill the virus, a special kind of white blood cell will bump into the virus and recognize that it does not belong. Armed with a description of the invader, these white blood cells go to the thymus gland and tell it to make a white blood cell which will kill the invader. The thymus starts immediately and in about two days releases the new white blood cell. These white blood cells then eat the virus in the process called phagocytosis. If you have already had the disease caused by the virus, or if you have been vaccinated for that disease, you already have the white blood cell which eats the virus. Therefore, when the virus invades your body these white blood cells multiply and immediately destroy the virus. This is why you cannot have most viral illnesses twice. However, as mentioned previously, the virus which causes the common cold changes so rapidly that previously made white blood cells may not be able to do the job. There are an estimated 5,000 mutant forms of the virus which creates the common cold. If you had had a cold from all 5,000, you would be immune to the common cold. This is not a likely, as the average person may have no more than 50 colds in their lifetime.
The AIDS virus, called HIV, attacks this immune system and destroys its ability
to do its work. Therefore, the person will die from things such as cancer or
pneumonia which would ordinarily be stopped by the immune system.