What is a virus?
The microbiological society describes a virus as the smallest of all microbes. With some, they are so small that 500 million could fit on to the head of a pin. They are unique because they are only alive and able to multiply inside the cells of other living things. The cell they multiply in is called the host cell.
A virus is made up of a core of genetic material, either DNA or RNA, surrounded by a protective coat called a capsid which is made up of protein. Sometimes the capsid is surrounded by an additional spikey coat called the envelope.
Under a process called Lysis, virus particles burst out of the host cell into the extracellular space resulting in the death of the host cell. It is this that causes the damage to the host organism, and the symptoms experienced, when many cells are killed. Once the virus has escaped from the host cell it is ready to enter a new cell and multiply.
An organism, if exposed to the virus or a similar version may have a degree of immunity as it has the genetic pattern to develop antibodies to fight the viruses and restrict their spread. The danger is when a novel virus is discovered, as organisms will have no pre prepared defences to these.