A virus is a tiny submicroscopic viral agent that duplicates only within the living cells of a living organism. Like a bacterium or a virus, viruses affect all living cell types, from plants and animals to microorganisms, such as bacteria and archaeans. There are over 100 different types of viruses in the human body; most are harmless, however, there are a few that can cause severe illness. Most viruses are copyists – they replicate themselves. This is called replicate virus or host virus.


Most viruses produce two types of genetic information, ribonucleic acid (RNA) and genetic codons. The genetic information contained in the DNA code of an infected cell acts as the guide for the transcription of viral RNA into actual viral proteins, thus causing the infection. However, sometimes there is a mutation of the DNA sequence, which can result in a temporary abnormality of the viral proteins, resulting in the formation of abnormal viruses.

Viruses are transmitted through the means of physical contact or indirect contact, such as by touching objects contaminated with pathogens. Some viruses can also be transmitted through the air when particles of infectious agent are inhaled. A large number of common viruses are co-transmitted between living organisms via chemical reactions that involve specific chemicals or enzymes. An example of this is the common cold or the flu.

There are many different types of viruses, and the way they affect living organisms is similar for each. For instance, we can divide them into two major groups: the pathogenic viruses and the non-pathogenic viruses. Pathogens are those that infect living organisms directly by contacting them. These include common house dust mites, dog and cat fleas, and certain types of protozoa, such as roundworms. Non-pathogenic viruses are those that do not directly affect living cell types, but rather affect the cells’ ability to function normally or in a healthy manner.

The nucleic acids upon which viral proteins are built are the most important factors in determining the nature and severity of a virus. A virus that affects a single host cell will be considered healthy, whereas one that damages hundreds of host cell will be considered a pathological virus. A virus that only affects host cell may produce only a mild disease, while a virus that destroys thousands of host cells and reproduces itself within those host cells can be considered extremely harmful. A virus that only affect one type of cell type, and does not invade other host cell types, are referred to as a host-based virus.

Many viruses have nucleic acid segments with viral RNA, ribonucleic acid, or DNA strands. When these segments of DNA are allowed to replicate, the segments form a piece of genetic material that becomes a virus. For example, many viruses that infect healthy cells reproduce themselves by using a protein called the nucleoprotein to replicate themselves. Some viruses have bits of DNA with viral RNA pieces that allow them to infect other cells. These segments of DNA called polyposis are recognized by a matching sequence of bases on the nucleic acid strand that allows them to reproduce.