A virus is a set of either DNA or RNA surrounded by a protein coat and a lipid membrane. Viruses must invade a host cell in order to replicate. Once a single virus injects its DNA or RNA into a host cell (for example, in your body) it begins to replicate rapidly and will create new viruses. At some point, the cell cannot contain the replicated virus anymore. The cell will then break open, which destroys the cell, and the newly replicated viruses will be released in the host body where the new viruses will attach to new cells. If 1 virus infected 1 cell and replicated 100 new viruses, the 100 new viruses (when released from the host cell) would infect 100 new cells. If each of these created 100 new viruses, there would be 10,000 viruses released next. The virus can quickly spread and destroy many healthy cells.
Viruses are infectious agents that are transported mainly by disease-causing microorganism called pathogens. Viruses have active DNA but do not meet the characteristics of a living organism until they are inside a host. This is because inside a host, a virus can respire, feed, reproduce, etc. Unfortunately, these activities have negative effects on the host's health. Therefore, the host has a protective barrier, the skin, and an immune system to safeguard itself from viruses.
How then do viruses manage to enter and attack the host's health? Viruses enter the body by way of bruises or bodily fluid. They then begin to attack the immune system, particularly helper T cells. In the case of HIV, the virus binds to the receptors on the surface of T helper cells and enters the cell at that point. The virus then ejects a copy DNA which enters the nucleus of the host's lymphocyte cell and becomes a part of the cell's DNA, permanently. Thereby effectively overtaking the host's cell completely. So, every time the host's cells experience mitosis (cell division and replication) so does this 'captured' virus cell. Eventually, the captured virus cell bursts open releasing thousands of new virus cells which continue to infect other T-cells. This eventually weakens the host's immune system over a short period of time.
A virus contains antigen which attack the immune system. When there is an antigen (virus, bacteria or foreign subject) in the bloodstream, the immune system releases anti-bodies. These fight back with white blood cells to consume the antigen. These are very clever because they remember what antigens they have come across so when they next come into the body, the effects may not be as strong because the anti-bodies know how to deal with them.
A virus is a foreign substance/agent that replicates in the body of a living organism via the cells. The usually have genetic material made from either DNA or RNA, and a protein coat that protects these genes. They are transferred via vectors, air, exposure to contaminated liquids (HIV) or contact with infected person.
They work by injecting their genetic material into the host cells. Once inside the cell, the cell begins to make the proteins coded in the viral DNA. The cell with viral DNA then goes trough the cell cycle and multiply as a result increasing the virus in the body and infecting the other cell.