There are many types of viruses, many of which include numerous subtypes. The virus is a very small obligate intracellular parasite that requires a living host cell for replication. The need for viruses for living tissue complicates any laboratory to grow or test viruses. When it is extracellular, a virus particle is called a virion. It consists of a protein coat, or capsid, and a core of either DNA or RNA. The protein coat comes in many shapes and sizes. The nucleic acid content provides one method of classification of viruses, along with the form of DNA, for example, a single or double strand. Most viruses contain DNA. A retrovirus suc
When a virus infects a person, it attaches to a host cell, and the viral genetic material enters the cell. Viral DNA or RNA takes over control of the host cell, using the host’s capacity for cell metabolism to synthesize protein and begin the production of many new viral components. The new viruses are assembled, then released by lysis of the host cell or by budding from the host cell membrane, usually with the destruction of the host cell- and the new viruses, in turn, infect nearby cells. However, some viruses remain in a latent stage; they enter host cells and replicate very slowly or not at all until sometime later.
One type of virus exists in many similar forms or strains, and viruses tend to mutate or change slightly during replication (e.g., the cold or influenza viruses). Both of these factors make it difficult for a host to develop adequate immunity to a virus, either by effective antibodies or by vaccines. Because of their unique characteristics, viruses are difficult to control. They can hide inside human cells, and they lack their own metabolic process or structures that can be attacked by drugs.
Certain intracellular viruses may also alter host cell chromosomes, thus leading to the development of malignant cells or cancer. The human papillomavirus HPV is considered to be a major cause of cervical cancer.
source: Pathophysiology for the Health Professions
