A virus is an infectious organism that needs a living cell to reproduce. It interrupts the normal functions of the host cell, eventually killing it. When the cell dies, it releases the viruses, which go on to infect other cells.
Some viruses do not destroy the host cell but alter the functions of the cell. Some viruses leave their genetic material in the host cell, where it remains dormant till there is some disturbance in the cell, and it may then begin replicating again and cause an infection.
Viruses spread through various media. They can enter our body when we swallow or inhaled. While some are spread by the bites of insects and other parasites (carriers like mosquitoes and ticks), some are spread sexually. If the body successfully fights the viral attack, some WBCs remember the invader and can quickly respond to a subsequent infection by the same virus. This response is called immunity.
Common viral infections are:
- Common cold
- Chicken pox
- Severe acute respiratory syndrome (SARS)
- HIV infection and AIDS