Most of us in one time of our lives had experienced being infected by a virus. Chickenpox, Measles and the common colds are a few common examples of diseases caused primarily by viruses. The most common viral infection in both men and women and in any age group is the common cold. The most feared and most controversial disease caused by a virus is the Human Immunodeficiency Syndrome (HIV). What really is a virus? What's in it that makes us sick?

Viruses are intracellular organisms that are made up of either DNA or RNA. These organisms dwell inside the cell and destroy it. Viral infections are self-limiting. This means, that these diseases will disappear in a specified time. Viruses are not killed by antibiotics or antimicrobial. That's why these medications are not given for the treatment of such diseases. Instead, medications are given to relieve the symptoms of the disease.

Here are the general characteristics of viruses:

* Small infectious units (measured in nanometer; about 20 - 300 nm in diameter)
- Electron microscopy is required
- Parvovirus - smallest animal virus
- Poxvirus - largest animal virus
* Obligate intracellular parasites
- Do not grow on artificial culture media
- Grow on: Tissue culture, Animals, Chick embryo
* Contain only one kind of nucleic acid--- DNA or RNA
* Viral components are assembled and do not replicate by "division."
* Not sensitive to antibiotics

A virus is composed of a nucleic acid genome (RNA or DNA), and a protective protein coat called the capsid. It may or may not have an envelope made of lipid derived from the host cell membrane. Virion nucleocapsid is consists of many structures. These are the icosahedral structure, the helical structure and the complex structure such as in the case of the Poxviridae which causes Smallpox.

Most DNA viruses have double stranded DNA except PARVOVIRUS and most are icosahedral except POXVIRUS. Here is a list of the DNA viruses:

• Adenoviridae
• Herpesviridae
• Poxviridae
• Papoviridae
• Hepadnaviridae

Most RNA viruses have single stranded RNA except REOVIRIDAE. Here is a list of the RNA viruses:
* Icosahedral
- Calicivirus
- Picornavirus
- Flavivirus
- Togavirus
- Reovirus
- Retrovirus (HIV, HTLV)
* Naked
- Picornavirus
- Calicivirus
- Reovirus

Viruses replicate by first, recognizing the target cell, then it attaches to it.
Penetration into the target cell follows the attachment. Inside the target cell, the next step occurs called the Uncoating. After the major processes of viral replication takes place, the advance stages occur in this sequence:

First, there will be a Macromolecular synthesis which begins with the early messenger RNA (m RNA) and nonstructural protein synthesis of genes for enzymes and nucleic acid-binding proteins. Then, there will be replication of genome followed by the late messenger RNA and structural protein synthesis. The first stage ends by the Post-translational modification of proteins. The first stage is then followed by the next three stages of viral replication. These are as follows; 1) Assembly of virus, 2) Budding of enveloped viruses and 3) Release of virus.

Viral infections are transmitted through different modes. The most common route is the oral-fecal route of transmission. The perfect example for this mode of transmission is the Rotavirus which is the most common cause of viral diarrhea in infants worldwide. Some viruses are transmitted by droplet spray and in the air. An example of these viruses is the Corona virus. A virus can also be transmitted through blood transfusion such as the Hepatitis C virus. Then lastly, a virus can be sexually transmitted, or transmitted by body fluids and secretions such as in the case of the Human Immunodeficiency virus or HIV which is the virus that causes AIDS.

Viral infections are diagnosed by direct physical examination. There are some viral infections that produce specific skin rashes that may help to its diagnosis. Some viral infections manifest through inflammation or swelling in the lymph nodes. Another diagnostic step that can be used is doing a Virus Isolation wherein the virus can be seen through microscopy. Viral infections can also be diagnosed through Serology and Molecular methods.

Although most viral infections are self-limiting, there are some viral infections that need to be treated with Antiviral drugs. Antiviral drugs are class of medications used specifically for treating viral infections. Unlike most antibiotics, antiviral drugs do not destroy their target pathogen; instead they inhibit their development.

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