What is a virus? A virus is simply an infectious agent which only replicates within a host cell. Viruses may infect various types of living organisms, such as plants, animals, and bacteria. The best way to avoid viral infections, however, is vaccination, which also administers an inactive vaccine into an affected person, so as to boost the immune system of the person to fight against any future attacks.
The most common type of virus is the Human Papilloma Virus, or HPV, which can cause genital warts in those who are infected. Other viruses include the Epstein-Barr virus, or ABC virus; hepatitis B virus; shingles; and herpes virus. When a virus copies itself and spreads to another living host, it is called replication.
Plants and animals make proteins, such as chloroplasts and ribosomes, but they cannot reproduce themselves, at least not in the same way as viral infection can. These cannot be spread between plants or animals, nor can they reproduce by means of their proteins within living host cells. This makes plant viruses different from bacterial viruses: Since plant viruses lack protein, they can neither replicate nor survive in living hosts. Since plant viruses lack a protein coat, these viruses cannot be transmitted via plants or animals (though they may be able to infect cells that are infected with plant viruses).
However, some viruses do make themselves multiply inside cells. Examples of these types of virus are HIV and the retrovirus that causes AIDS. When a virus multiplies inside a cell, it takes over the DNA of that cell and changes its function or structure (usually making the cell unstable). This process, called “turning the cell off,” is how HIV enters the body and causes AIDS.
Some viruses infect only a single cell. Examples are herpes and cytomegalovirus, which respectively cause infections in the cells of the skin and genitalia. Some viruses infect multiple cell types: examples include HIV, SARS, and hepatitis. Whether the virus makes any type of changes in the host cell remains to be seen; researchers have yet to discover all of them.
Viral infections may be spread through the use of particles that contain viral RNA or DNA. This is why cold sores often occur on the lips or hands. The particles that cause cold sores to spread rapidly are the same particles that allow viruses to replicate within living host cells. The research on these particles is currently underway; scientists are trying to identify the exact mechanisms by which the particles cause the viruses to replicate within the host cells. It is currently thought that cold sore viruses infect the cells by means of viral acid, but this theory remains unproven.