From the relatively benign common cold and cold sore to the lethal diseases smallpox and Aids, the microscopic virus has a lot to answer for. These parasites only come to "life" once they have infected cells of plants, animals and bacteria, because only then can they reproduce through using their host.
Surrounded by a protein coat, they contain a core genetic code or viral genome of DeoxyriboNucleic Acidor (DNA) or RiboNucleic Acid (RNA), which is used to replicate the virus.
They can be passed through direct contact or by a carrier, such as an insect. Once inside the host cell the virus causes it to begin manufacturing the proteins necessary for virus reproduction. Then the virus is secreted out of the host cell to begin infecting other surrounding cells.
Because the virus uses the machinery of the host cell it is very difficult to kill, but the most effective approach is vaccination, where a weakened or dead virus is injected into a person to stimulate the production of antibodies, which can then kill off that type of virus.
The name H5N1 (pictured below as a coloured transmission electron micrograph) refers to haemagglutinin type 5 - the types of antigens (usually proteins) that are found on the surface of the virus, which bind it to the cell it infects - and neuraminidase type 1, which injects the viral genetic material into the infected cell.